SHOTGUN APPROACH!

Did you or your neighbors remove or cut back trees or shrubbery?
How bad a pollen year was it in Virginia? Was it milder than usual?
How much usage did your pool get?
Did you keep chlorine levels higher this year than usual?
Did you do an unusually thorough job cleaning your filter at the end of last year?
Did you have your pool open a shorter time?
Did you have a colder summer there, too?

Just throwing out some ideas.