Confused, what could have happened?

Okay i was at my guy friends soccer game he's 15 and he got tripped and someone stepped on his knee really hard when it was at a weird angle and there was this pop cause i saw and heard it all....he grabbed his knee but then after about 2 minutes he was still on the ground and he started to get out of it and then he fainted so his coach called 911 cause he didn't know what was wrong...t turned out he has a torn acl and meniscus or whatever its called lol....but why did he pass out like that it was weird any ideas of what it could of been???