They hear a sound, a loud rustle in the bushes.
Bob is a pessimist. He thinks the worst, believing the sound was a Sabre-toothed Tiger and feeling he'd better move on through the woods pretty quickly to his usual safer hunting spots. He legs it.
Bill, on the other hand, is an optimist. He thinks more positively about the situation and tells himself the rustle in the bushes wasn't a hungry predator, it was just the wind. He doesn't leg it. He gets eaten.