Just watch the scene it skips to. Background for those of you who don't watch the office: Tim (the first guy you see) is very good friends with Dawn (the blonde receptionist). Dawn however, is engaged to Lee, the guy who walks in with the bag.
The exceedingly awkward moment you just watched has two underlying themes of awkwardness I want to explore. The first is being the friend of someone with a boyfriend (especially when he comes strolling around). Now perhaps (although I doubt it) you are genuinely friends with this girl just because you like to hang around her and you have no hopes or desires to someday advance a romantic interest. Even if that's the case, when you're chilling with her, there's still that awkwardness when the boyfriend comes strolling by. When he makes his appearance, you have to try overtly to make it seem like you have no romantic interest in the girl and let the boyfriend know that you're not trying any funny business or messing around in his territory. (You'll notice in the video, Tim looks away into space awkwardly acknowledging it was her boyfriends time to talk with her in semi-private and he was totally ready to back off). Now if you are romantically interested in the girl (as Tim clearly is) it makes things much more awkward for very obvious reasons. There is always the tension: the subtle unspoken issue of male dominance (who will prevail?) that can never be talked about but is always there in such a situation. Whenever you run into the boyfriend on his own (if you have the misfortune of doing so), your interaction will be cordial, yet there's always the obvious fact that you connect 10x more with his girlfriend than him. In his eyes, this either makes you gay and not a man's man or someone who's clearly trying to creep on his girlfriend. In your eyes this is just uncomfortable and awkward.
Now the second reason the situation in the video was awkward is a more common form of awkwardness and one which interests me much more than the first. I wish I could have found a video of this situation in isolation but the best I could find was a conglomeration of awkwardness so I'll try to be distinct as I can in breaking it down. This situation is when you are good friends with someone and a friend of theirs (whom you don't know well at all) enters the room. Your good friend then leaves the room for some reason (say, "hey shoot give me 5 minutes I gotta pick up my laundry downstairs") leaving you forced to interact with this person, all the while knowing the only reason you both are there is for the person who has left the room. It would be rude and seemingly desperate if you were like "hey I'll go with you!" or simply ignored that friend of your friend. That would make it seem like you are incapable or unwilling to try to make friends with this friend of a friend (which you very well might be, but you can't let anyone know that, or it's awkward, and kind of rude).
The best is that moment when Tim is desperately trying to think of something to say, to think of some way to connect with Lee, after the one thing they had in common left (Dawn). It takes him about 10 seconds before he can come up with, "What's in the bag?" Wow, "what's in the bag?". You know the shit has hit the fan when you start stammering out stuff like that to fill the silence. Although Lee is kind of a dick by just being like "alright see you later" and cutting off Tim's question (albeit a banal and pathetic question), in a way it is respectable. He's not bullshitting around: he is there waiting for Dawn and doesn't really want to socialize with anyone else so he'd rather wait in the car for her than shoot the shit with forced small talk for 2 minutes with Tim, whom he's not friends with. I also love how Tim tries to play it cool in front of the camera afterwards and act like it wasn't even awkward (by carefully investigating the objects on the desk, as if noticing them for the first time...)
A few weeks ago at a party I was talking to a friend of mine and he introduced me to this girl I had never seen before. After having known her for all of 20 seconds he was then like "OK I'm gonna go to the bathroom. I'm gonna leave you two alone- you can either decide to go your separate ways if you feel too awkward talking or try to bond and get to know each other. Your choice. 3, 2, 1, bye." And then he left. I was awestruck. He just brought the elephant in full view for everyone in the room to see. Although it maybe should have been relieving and refreshing to have someone point out this common situation, I still found it quite awkward after he left. I guess it was mainly awkward because he had rendered me incapable of pulling the usual excuse I might have pulled in such a situation. I couldn't say something like "I'll go with you" (to the bathroom) or "I'm gonna get another drink" because after him saying that, it would be so obvious I just wanted to get away from the awkwardness of the situation. It's just so awkward having the usual BS small talk conversation but meanwhile both knowing you would be having a much more interesting, fun, and relaxed conversation with your mutual friend and the only reason you're talking to each other in the first place is because you both wanted to hang out with him. If you're curious, we ended up shooting the shit for a few minutes and then going our separate ways (with me pulling some line like "I'm gonna go look for my friend"). Shameless.
This situation can be most emphasized in a college dining hall. Let's say you're sitting and eating with one of your good friends and one of his friends whom you don't know very well (or perhaps are just meeting for the first time) sits down with you guys. OK great, all is well- conversation flowing nice and smoothly. Perhaps you would have enjoyed dinner more if you were alone with your friend, but oh well, don't want to make this kid sit alone. Now suddenly your good friend realizes "shoot I have to go to this [insert club/activity here] meeting. I'll catch you guys later." Now you're stuck eating dinner with the person you never would have sat down with in the first place. You both know that you never would have sat down with each other if it weren't for this mutual friend. You can't get up and leave even with an excuse (your plates are both still half full of food) because that's just rude and so obvious that you just only wanted to socialize with your good friend and are too lazy and antisocial to make an effort with him.
There's a moment on Seinfeld (I believe it's "The Dog" episode) where after Jerry, Elaine, and George (three of the main characters, who always hang out together) have been hanging out and Jerry leaves the room, it becomes very awkward between Elaine and George, who are left alone. They suddenly find the only way to make conversation is by making fun of Jerry or talking about him. Despite this brief solidarity, it is in this episode that they realize they actually weren't really friends to begin with and were just both good friends with Jerry. In his company, they just assumed they were friends with one another, but as soon as Jerry left them alone (which I think is the true test of being friends) their relationship was shown for how superficial and lacking in substance it really was.