If a dog is a living organism, then fleas -- as they gain the ability to jump to other dogs -- would be it's reproductive organs. Which means that a dog with fleas jumping off is just beginning to reach sexual maturity.

Organisms hitting puberty are notoriously moody, weird, and confused, alternating between bouts of idealism and self-destructiveness. Looking at the state of dogs crawling with fleas, I'd say the flea-bitten-dog hypothesis, at least in this respect, is holding up.