It is also a learned behavior once they realise it get's them the fawning over they want during drop off and after coming home. Don't always be so quick to blame the groomer because it's often wrong. Groomer's get blamed for a lot of issues that were in existence before the visit as well, but that isn't what your vet hears and it's easier to balme someone (ear infections are a biggie with that one). Yes, there are bad groomers, but it just as often has nothing to do with them.
