Fish are not smart- that is why they are...fish. They do not think rational.

They will do it when:

They have lost a couple of big hands

Rationale: 1) Its my turn to win, 2) I want to quickly win back what I lost.

They believe that someone has bluffed them off a hand

Rationale:

1) I will show them what a bluff really is. 2) I will not be bullied!

They are on a losing streak, bad negative variance

Rationale:

1) I have lost so much, what does another buy-in matter now?

2) My luck is bound to turn around- now!

3) Poker is rigged. I have lost with Aces and Kings, maybe I will win if I play those crappy cards myself?

Hard for me to think like a fish, but maybe they think along these lines??