I think Norrish got it wrong. In his type I reaction, the photoinitiator (PI) breaks into 2 parts,
while in his type II reaction, the photoinitiator stays together as one molecule. Wouldn't it make more sense for the Type I to be the reaction where the PI stays together as 1 molecule and a Type II to be the reaction with the PI breaking into 2 parts?
But who am I to criticize? He's got a Nobel Prize, and I? Well I know how to spell "Nobel Prize".
I imagine that this was simply the ordered in which he discoverd the reactions.