Yeah, jake_torres68 is onto something with rain exposing weak seals. But I've read that cold rainwater can make some seal plastics brittle way faster than normal aging. If the material isn't graded for quick temperature drops, it might crack from the chill alone, not just from water getting in. That could explain why some seals fail even if they looked solid before the storm.
Actually, it might be more accurate to say wet weather shows you which seals were already weak. A good seal should handle rain just fine. The water finds tiny cracks or dry rot you couldn't see before. So the rain isn't causing the failure so much as exposing it. That's a key difference when you're trying to fix things for good.