Possibly. There are some pretty wild temperature swings. I'm not a geologist, but I think rocks usually fracture along the plane of their crystalline structure (as it's the weakest bond)

Let's propose that these eroded boulders have a different crystalline structure due to the way they were formed / modified by groundwater etc., then they maybe they are fracturing in a piecemeal fashion, and basically crumble away due to the wild temperature swings.