It may also have something to do with how you've cut your binding. Bias binding will stretch and do all kinds of wonky stuff if you use it on straight edges. Save that for curves.
When you cut your binding, try cutting it on the straight grain the length of your fabric instead of the width. The width has more stretch than lengthwise.
Another trick is to starch the "crap" out of it when you're folding and pressing it before attaching it.
"If it's wild, tame it with starch!"