I think he was beaten and stabbed by 16 dudes.
I don't claim to know anything about prison culture, but it seems like if 16 dudes decide all at once to beat you and stab you, you'd THINK you would've done something to provoke it, right? Can you imagine 16 dudes just eyeing the courtyard going, "now, which of these dudes should we beat up? Hmmmmmmm..... How about that guy?" I mean for 16 of them there must've been a reason. Could be a really fucking stupid reason, but I'm sure a reason none the less.