What causes seasonal doors and how do you prevent it?
The condition you outline is not uncommon. It usually results from humidity changes. Typically, the tolerances were a bit too tight when the house was built, not exactly squared or the door framing was not secured as tightly as it should have been. 

Here are a couple of things you can do now to mitigate the problems.

Replace the center screw of each hinge on the casing with a longer screw. A 2 1/2 to 3 inch screw will be sure to penetrate the Jack and Stud, thus holding the door casing firm to the framing. Make sure all the other screws are snug.

Check the casings for a secure attachment to the underlying studs. If you press against them hard with your hand and there is any movement, you may need to add a few longer finish nails to secure them to the studs.

Trim the doors by sanding or planing them at the time they fit the worse.

Some one or combination of the above should permanently fix your problem.