What could be the cause of a large and bleeding sore on roof of mouth?

My dad suddenly noticed a large, flat, red sore on the back of the roof of his mouth. It started bleeding and now he’s at a walk in clinic. What could be the cause of such a sudden outbreak? He did start a new medication, but I’m unclear right when he started it. But, he said in the meds, it didn’t list a mouth sore as a side effect, so who knows. I just hope everything is alright with him. he’s on a couple medications.

so many things -rinse with peroxide, brush & floss, and go to an urgent care.