I haven't thrown up after squatting for a while, but I think it might have to do with excess lactic acid and too much blood pumping too hard (very scientific logic that).
I often have to just sit and rest for a few minutes before I can leave the gym though.
Last time I threw up I walked out of the gym after squatting completely rooted and met my girl, who was meeting me after doing some shopping (my gym adjoins the mall). She thought I looked pale and dizzy but I told her I'd be ok, before projectile vomiting about 3 litres of water and various just past her - lucky she had quick reflexes. She got me a Coke because I felt like i was going to pass out and that made me feel better, until I threw that up at the front doors to the shopping mall. I got some "admiring" looks from passers by.
IronMike