Why do inflatable camping pillows slide around at night?

It's the most common complaint across customer reviews for air pillows: slick fabric on a slick sleeping pad equals slippage. Pillows with brushed or fabric-topped covers, or hybrid foam tops, are reported to stay put better. Some campers clip the pillow to the pad's pillow lock, tuck it into a hood, or set it inside a stuff sack to anchor it.