What Danimal said - Mine does it after washing. I guess if we drove them after we washed them then the pad would have a chance to dry but when you put them away wet you get the rust.
Mine has done it since day one after I wash it, I put it back in the garage so its clean for the Sunday Cruise In I go to. I drive it to work most days and it doesn't pop, only when washed and put away, nothing to worry about.