Softis

I know I'm a bit late, but I want to confirm and elaborate on the previous answer. Taking the Crab Spider set on any build will cause the backpack to stay on you, even if you switch to a build that does not build Crab. Picking the Crab set seems to apply an invisible modifier on the player that forces it to appear on your back, which is pretty understandable since the Crab powers rely on the backpack. This effect is applied no matter which slot you build Crab on. If you no longer wish to wear the Crab Spider backpack, you will have to respec your Crab build and change it to Wolf/Bane.