alright so I did this earlier

and I'm pretty satisfied with it to be honest, but there's a little issue with the rig in the back
when I position it like this:

this weird squished stuff happens in the back

so, mostly for future reference, what's causing that problem, and what could I do to fix it?
also I'm using CAT in 3DS Max if that matters. I figure most skeletal animation things are pretty similar though