It's poor and inadequate prepping of the bumper before painting.

Rubber bumper covers really need to be baked in the booth for a cycle to hat them up and help push out some of the mold release agents then the covers nee to be treated, cleaned, prepped and sanded properly to assure proper adhesion. Most shots dont do this, but a good one will. If you would like a recommendation on a good shop in your area shoot me a PM.

Good luck. And for the record the shop that did your wife and daughters cars, so long as they are reputable and legit, they should offer a life time warranty and repair their vehicles with no issues. If you would like info on these shops you can let me know in a PM who they are and I will provide you with what(or who) I know.