Because your manager might have been dealing with something privately, and didn't feel like doing something fun, but had to because the Gods Of Corporate decreed it so.

And so, an act of rebellion against a shared reality that forces you to have fun on schedule when it's time for the quarterly offsite.

I dunno watching office space with my team doesn't seem like the worst way to spend some time together. Maybe just reading too much into it?

Office Space has become a part of the corporate culture's shared language. You are out of the loop if you haven't seen it, and it wouldn't surprise me that a lot of people are introduced to it in a work-related environment (as I was).