When the wind blows over your house, the air pressure changes because of the "Bernoulli effect". The Bernoulli effect says that pressure becomes lower when flow is faster. It's this effect that sucks loose objects out the car window when you open it while driving; it also creates the lift that allows an airplane to fly. As the wind blows over your house, the Bernoulli effect lowers the pressure at the top of the vent stack. This creates a slight suction throughout the plumbing system, which pulls against the water sitting in the trap in the toilet bowl. As the wind strengthens and weakens, it produces more or less suction, which causes the water in the bowl to slosh.