The 50 lb pressure would also seem to indicate that they were LR C tires, not D.
Not all tire issues are the result of the make/model tire. Aside from damage from overloading and driving faster than the tire is rated for, sometimes stranger things can happen. I got lucky this summer with a flat. Pulled into a gas station to get gas and noticed a tire on the trailer looked low. As I filled the car with gas, it got visibly lower. I figured I must have run over something while getting off the highway so finished filling and pulled to an out of the way location, swapped it for the spare, and continued on my trip. (Very lucky how it happened.) When I got back home after the trip I looked the tire over and couldn't find anything wrong with it aside from it not holding air. I took the tire to a local tire shop and it ends up it wasn't the tire - the rim had cracked. The tire was fine and is now serving as my spare, on a new rim.
