There could be a couple of reasons for this. Sometimes when dogs are boarded, they will start to "hold it" because they want to keep their area clean. When they realize that this is impossible, they stop seeing their area as their own and are forced to soil it. This behavior continues at home.
Aside from that, the surgery can cause some inappropriate soiling because of the pain that is associated with it.
I would see the vet for a urine culture to rule out a medical issue and then go back to the basics with housebreaking if this proves to be false.
Let me know if you still have questions.
