Collapsed rear axle boots on both sides of the car

One edge of the round, multi-ribbed black rubber axle boots (CV boots??) that mount against the transaxle have collapsed exactly the same way on both sides of the car. They were fine when I last looked a couple of hundred miles ago. When I try to squeeze the sides of the ribs in an attempt to pop them back, they won't stay. They just go back to the "sucked in" form. It is as though there were some sort of partial vacuum inside. Has anyone ever seen this before? Could there be a vacuum inside? If so, why? I am planning to loosen one of the clamps in case there is a partial vacuum in the hope of restoring its proper shape. Weird! I never seen anything like this before. Anyone have any ideas?