Why is my dog biting on my moms pants and not letting go?

My family is fostering a German Shepherd puppy, which is almost the size of a normal shepherd. Sometimes, she will bite onto my mom's pants while she's wearing them, and not let go if I tell her no or tug at her collar. And when the puppy does do that, it scares my mom because why the hell is it doing that for no reason? It sees my mom every day, so it's not like she's a stranger.