Unofficially it's called "clipnosis". As kittens, cats are picked up by their mother on the back of the neck, when this happens they instinctively go full submissive and their body relaxes itself. Often times this stays with them to adulthood, you can usually force the same effect by pinching the back of the neck of your cat. If the harness or outfit they're wearing causes pressure in the right spot they can also react that way.



Or they don't like what they're wearing and are simply laying down like, "f*** this, I will not amuse you".