Maybe it's the saddle. I'm seeing people reporting that the Brooks Cambium saddles are abrasive to trousers. It's on all of my bicycles - the Brooks C15 saddle. Now I have to find an alternative that is the same thing but with a smoother surface. A leather covering would probably work, but, I'm not seeing any, so far. The mail order bicycle retailer sells waterproof covers for leather saddles - not what I'm looking for at all. I guess they want people to buy expensive leather saddles, rather than leather covers for existing favorite saddles.
I don't know what a trouser is. What's a pant?
Mysteries of the universe...