Friction, sun, sitting in chairs, etc. It happens over time.

I have seen plenty of *** crack in my day thanks to worn shorts. One guy thanked me for letting him know.

The same happened to me during Cycle Oregon in 2002. A woman riding behind said “Your shorts must be really old.” They were, but I had to wonder how she knew. After a minute or so it dawned on me. At the end of the day’s ride I inspected them. Into the trash they went.

Mine typically wear thin on the tops of the thighs even though that area doesn’t contact the saddle. Also, are you putting DEET on them? That’s not a good thing to do.