They move because there is little contact between a couple of small spots on the skids and the deck. Non-skid paint really only helps prevent slips by people walking, it's far too fragile to do much to stop something as heavy as a helicopter. The worst are wavy decks, which aren't as flat as they should be. You end up getting about 2 or 3 square inches of contact between the skids and the deck, even on a flat deck. I've had helicopters spin on me during cranking, and one turned a full 90 degrees before I could get up to idle. Bells, especially the 206/407 series, weathervane very strongly, and given a strong wind, in the 30 knot range, they're going to try to turn into the wind. The already weak tail rotor authority may not be enough, especially at idle, sometimes not even at 100% N2.