He didn't see it,imagine it like you can take 700 pound of force when ur not aware but if you see something coming you can take 1200 before the lights go out.
When you see the old sparring videos of as its crazy to see him with n o headgear letting people punch him full on,and the rivera fight looking straight in his eyes and smiling as rivera unloaded before finishing him.
In all honesty age is a big factor,fighters can age a lot between fights the bonnar fight was like 8 months before the Weidman fight and that's a lot for an old fighter.