Mike, I don't know for sure. Most of the ones I've ever seen were attached to the sides of rocks or under slabs or associated with dead coral heads (and the same goes for V. ceramicum and V. tubiferum in the Pacific), in areas that were primarily hard substrate with only a limited amount of sand. It could be that something is flipping them, but can't eat them due to the thick shell and adequate operculum, or it may be that they have a hard time reorienting themselves after being flipped or detached by waves or other animals. If that's the case, as it is with some of the Astraea/Astralium species, they may stay inverted for long periods (or forever) if they get flipped upside down on the sand. If you aren't moving much material and looking under it, that might account for most of the ones you see being upside down? There are a few other species that tend to turn up on the sand, inverted, quite a lot, and I suppose for the same reasons; but you can usually find large, living and un-inverted populations nearby if you are finding dead or stranded ones scattered around on the sand. If you're finding them inverted on hard substrate, even after calm periods, it's hard for me to give you a good guess what might be causing it.
Cheers,
Don