Been there done that. Had a friend that wanted me to test ride a fatboy was looking at for his wife. He said the thing would just fall into turn and was very hard to get it righted. He thought it was neck bearings. I drove it about 30 feet and told him to check the tire pressure. It was 20 psi. Only about 16 psi low. So yea, I'm thinking low tire pressure caused it. Be that because of a puncture, or just not checking it.