Sachin complained to Bucknor about the light. Just before that over or maybe a little before that, he had quite a long chat with Bucknor and the great Bucknor didn't seem to do anything about it. Then just before the ball on which he was declared out, Sachin made it very obvious to Bucknor that he can't see the ball at all.
I think Bucknor got pissed off with him for doing that. Note that Bucknor's been pissed off at Indians before for things like appealing. So when Razzaq appealed like an ass for nothing, Bucknor said "Ah, here's my chance....off you go dude..." and the rest is history.
Bucknor might have been one of cricket's greatest umpires...but he shall not be remembered like that if he continues his shitty umpiring.