This phrase is often used by pit bull owners and advocates after a pit bull unpredictably and unpleasantly attacks. The  intention is to minimize the attack by saying that the dog was “just playing”. For instance, after a pit bull attacked UFC fighter Kyacey Uscola in May — one bite puncturing through his penis — a pit nutter said the following:

“[Pit bulls] aren’t anymore aggressive than other breeds of dogs, but simply have a stronger bite which leads people to think that they’re attacking when to them they’re simply playing around. – doomrider7

Chet 99 on, who often posts pit bull mauling stories so that forum members can respond, posted a story about a pit bull that attacked its owners, sending one to the hospital. It was reported the next day that the dog was euthanized because of the severity of the injuries.

The couple was lucky their pit bull was just playing.
“He says there appears to be no reason the dog attacked them.”
NOT an attack. The pit bull was just playing. Sheesh, what’s the big deal? – ARepublicanForAllReasons