Happened to me last summer with my 15" rims (replaced both the axle and wheels with a higher capacity one three years ago). Pulled off the highway to get gas 150 miles into the start of the trip. Checked out the trailer like I always do, and thought a tire might be a little low. By the time I was done filling up, that tire was almost completely flat. Changed it out, finished the trip (no other issues), took the flat tire to the shop and they told me the tire looked great but the rim had cracked. Just dumb luck it went while I was getting gas.
