That’s where the dog’s body decides that it’s platelets are foreign invaders and starts killing them off. They generate antibodies to their own platelets. These antibodies start killing off the platelets. Platelets handle clotting. They’re half the way a dog clots, but, fortunately, there’s a whole other pathway. So they don’t USUALLY bleed out.
Have to figure out why and how the dog is killing it’s platelets and stop that.
Here’s a document on AITP
Here’s an 18 page flip book you can read online (free)