The first few days of the event, the temps were close to 100 degrees even after the sun went down, so a packed parking garage with no air moving was probably pretty brutal. And even when temps are normal, it's pretty stuffy -- I hated standing in there last year, but luckily it was the Pandora queue so I only endured it a couple times. But I remember the air was just super thick and uncomfortable. It was pretty loud, too. The sound reverberates off all the cement, and then people talk even louder because of that.
Also I imagine if you're sensitive to those kind of environmental conditions -- like if you're on the autism spectrum or something -- it's probably extra annoying.