Why does the non-skid backing of the bathroom rug stick to the floor?

I just pulled up my bathroom rug to wash it and the non-skid backing stuck to the floor. Most of it stuck to the floor, ruined the rug so much that I had to throw it out. Now, my back hurts from scraping off the non-skid backing with a putty knife. What gives? Are these kind of rugs flawed in their design somehow? Someone please enlighten me.
Thanks Sensible. I did however have one with the rubber backing.

Most have a cheap foam backing. Add moisture and different type surfaces and they stick. Get one with regular rubber (not foam) backing.

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