Foot Obsession

Sophie seems to have an obsession with feet. Whenever I walk around my apartment, she follows me and, after each step I take, attacks my foot by biting my big toe! What a strange dog!
Doyle, on the other hand, will only attack my feet at night while I try to sleep. All of a sudden, my foot will get a nice reminder that it is not supposed to move until Doyle is ready to get up! Did I teach them this strange obsession? How in the world did I get two dogs, both obsessed with feet?