Atmospheric pressure should be the same inside and out (unless the house is hermetically sealed) but the buffeting of the wind can cause large changes in pressure locally. In this case the wind is causing differences in pressure between the air on one side of your U-bend and the air on the other side. Just imagine the smell you'd have to suffer if you didn't have sink traps and the U-bend!

I would suggest that it is the same principle at work that alloews flight: the Bernoulli Principle.  Air moving quickly across the top of the vent pipe of the toilet causes a local decrease in air pressure inside the vent stack.  This causes the (relatively) high air pressure inside the toilet bowl to push the water level down in the bowl.

yes , toilets drains are vented to stop the "Head" of water dragging air down behind it resulting in a toilet that " glops" after you flush it , the farther away the toilet is from the vent the more it "glops"

the vent is nomally above the roof line , and any air presure diffence between the top of the vent and inside will cause the water to move