Don't know if you figured it out or not but it looks to be a weightmap issue. You would have to look at the weightmap of the trousers. It appears that some of the vertices are linking to the opposite legs causing that stretching issue.

Hi everyone
I have a problem with my character trousers.
It appears that when it spread it legs the trousers get flat and begin to stretch.
Like the legs are connected.
I don't know how to fix this so I really need you're help
Thanks a lot