Unexpected crack in d pillar plastic, how???

Hi, I have a 2000 ML430. Now I dont know how this happened, but last weekend I washed the car so I am positive this crack appeared in the last few days. I am referring to the passenger side D pillar, thats the one between the rear hatch and the fixed side window. There is a plastic capping which has cracked horizontally from one side to the other. Now I know I have not hit it, and there are no signs of anyone hitting it, no scratches, no dents, nothing but a hairline crack runnig across it. Has anyone else experienced this????