An attempt at an ExplainLikeImFive answer:
In a sizable volume of gas the air molecules have a lot of room to bounce around. Moving molecules and them hitting things is heat. So suddenly having a lot less room results in much more frequent impacts. Frequency of impacts is heat, so it becomes hotter. We'd sort of normally consider it the overall average velocity of the molecules, but if they never hit anything they never transfer energy, and aren't measured (or measurable). But when they hit things they transfer some of that energy and it's measured as heat as an aggregate.
So biggish volume of gas, suddenly in a tiny volume: huge spike in heat because all the molecules now slamming into each other and the people inside.
I'd say it would be very unpleasant, but it's so fast and so violent that exceeds the speed of human thought, so they felt nothing and just sort of stopped existing as corporeal beings faster than they could possibly comprehend the change in circumstances. They were. And then they weren't.
