Why does Diablo stop attacking at extremely low life?

I noticed that when Diablo's life gets really low (not sure the exact %, but it's below 5%) during the final stage of fighting him, he will simply stop attacking and sit there, usually in some strange "I'm dying" position

Is there a reason for this?

The only thing I've used it for so far is to res dead players before killing him, and to remove my weapons/skills and punch him (for the Punch Diablo achievement)

Solution 1:

It's just a hard-coded 'cinematic' death - my friends and I like to call it 'beating the loot pinata'. If he starts doing it, you've won - celebrate (and make inventory space for the terrible loot he's about to drop)!