Odd problem with clothes

So I’ve rigged my character and everything seems to be fine so I started to make some clothes for him, I’ve made a t-shirt, parented it to the armature and it works fine, then I’ve made trousers, parented them to armature and… I guess those pictures are self explanatory:

It happens to every move I do. What’s the problem? I can’t figure it out :expressionless: