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UV and heat, quite possibly. I've also learned that anytime anything aloft "gets bent" or goes missing, it is often from birds landing on it and breaking if off because it was not designed to be perched on.
Depending on the cost of the cups versus shipping and handling, you might want to order a spare set at the same time.
Only a webcam or wildlife camera is going to confirm or refute a bird problem. But if you can arrange some steel wire (i.e. piano wire) in the form of vertical whips that come up around the wind cups, that might discourage the birds.