This has happened twice lately at gigs I've been at.
First time was watching Godspeed at The Forum, they had just gone on and this guy a few rows in front of me completely lost the ability to function. He was fine one minute then gone, and no one could hold him up as he was like a dead weight, even though he was a little guy, and his eyes were open but glazed over. He needed two guys to hold him up and drag him to the back.
Then it happened again last night, also at The Forum, watching the Mark Lanegan Band. We were behind the sound-desk so it was hardly a packed area and there were a group of 5 or 6 people next to us. It's nearing the last 30 mins of the gig and I look round and the bloke who'd been standing next to me the whole time (not even drinking) just passes out. None of his friends knew what the fuck was going on as he was just leaning into them but steadily going further over to the point he was going to fall over. He was a big lad and it took a few of them to keep him standing, but again the eyes were wide open but glazed over and he'd lost all ability to stand.
What is this phenomenon? Is it simply to do with heat / claustrophobia? Bad drugs maybe? How do I know it's not going to happen to me next time I'm at a show?