Quark has an ability for you to sit on stairs, but with this mod enabled, it makes you crawl under the block.
Mods that could cause the problems: Quark, Ido, First Person Render 2, More Player Models, Mobends, Custom NPCs, Vanilla Fix, and Dynamic Stealth. I'm pretty sure the other 180 mods aren't causing this problem.

Should be fixed in the next patch, if not more testing will be done to see if there is a way to fix it.