Oh people are definitely disgruntled, but I get the feeling that's only a part of it.
I think it comes from the sense of isolation. I admit that this is purely my informal observation, but I see consistent trends relative to the amount of time people have spent in this job (or similar office environments) that correlate pretty well, in a toned down fashion, with the effects of solitary confinement. They get irrationally (passive) aggressive and paranoid. They stir up trouble because in this highly controlled and sterile environment, they seem to lack anything better to do. Their ability to connect with other people seems to atrophy (this is one I've felt a lot personally). The middle aged veterans of the office are almost all divorced and seem to have absolutely no life outside the office. They'll work as much overtime as they're allowed and will just hang around doing random stuff when they're not. One guy goes around with a pair of scissors and gently preens the plastic plant decorations as if they're his personal garden. Others just spy on each other (even rummaging in other people's desks when they can get away with it) and come up with excuses to spread petty gossip, ignite office politics, and file complaints against each other. The office veteran brigade on the shift immediately after mine targeted me once for reading books at my desk on my lunch break...
All the satire out there about corporate office environments are not very exaggerated. Office Space. The Office. Dilbert. Those eccentric and warped personas are mostly accurate depictions of the type of people you'll encounter in a place like this. The only major difference would be the employees have an unrealistic amount of freedom to walk around talking to each other, thus creating content for black comedy writers to work with. In reality, at least here, we've been through periods where we're officially unallowed to talk to the people sitting next to us. All of our work-related activity is monitored, and we're questioned on any inactive time spans. Younger workers like me turn to the internet to maintain their sanity. The older generations just go a little batty, I think.