evolutionary instinct of rebelling against threats, even if it's not entirely optimal.
I'd be wary of criticizing a decision heuristic's optimality immediately after explaining how that heuristic can often lead to victory. Precommitting to rebel against threats is a half-decent way of preventing threats, and it's hard to consciously muster a precommitment more convincing than pure instinct.
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.