If this were a mechanism that appeared under some kind of evolutionary pressure. What would that be? In which situations a normal mammal find its bones squished (but not crushed)? Or is it just a side effect of something else going on?

Good thing I decided not to turn into a mollusk!

The mandatory Bone Song[0].

0. https://www.youtube.com/watch?v=kHgRlsOvN3M

So in mice running doesn't do it but vice grips on the bones do?

How would that translate to lifting weights or other exercises?