Had transmission checked out

[gw33gp]

I had to take my transmission out to replace the slave cylinder that had a very slow leak when cold. I decided to take the transmission to a rebuild shop to have it checked out because it has almost 243K miles on it. It was working perfectly with no unusual noises with the exception of a very light kind of rattle only when cruising in first gear. I asked them to replace all the seals at a minimum and anything that that was not in good condition. I expected they would want to replace the bearings also. I was called to tell me fifth gear and synchro were bad and the fifth/reverse gear assembly should be replaced. He asked if it was popping out of fifth gear or any other problem or noises in fifth. I told him no and it was working perfectly. He said well we caught it in time then. I asked if the input shaft gear was in good condition and he said it was good.
What would cause the fifth gear to go bad? Is it just the high amount of miles with it being in fifth most of the time? I do tow in fifth with 31" tires but that has been only the last 2 or 3 years after having 4.56 gears installed. Before that I would tow in 4th with the 4.10 gears with 31" tires.