If these wheels are brand new, this probably isn't the cause, but just for completeness: Mavic's FTS-X freehub system rides on an oil-bathed bushing rather than the more common bearing assembly, and if that bushing is not maintained and kept lubricated it can wear significantly, leading to loose fitting of the freehub body and therefore the cassette. You can tell very easily if this is the case with your wheel: grab the cassette and try to wobble it off the central axis of the wheel. If there's any significant amount of travel, you're probably going to want to at least have a look at that bushing; you may need a new freehub. Details on how to get into the freehub to have a look are here:
http://www.mavic.ca/sites/default/fi...oad/FTSX_0.pdf
The manual warns in scary terms to ONLY use the super-secret proprietary mineral oil that Mavic will sell you for a hojillion dollars per fluid ounce; I just use 10-weight fork oil, which has so far completely failed to disintegrate the hub, despite the warnings.
Even if this isn't the specific issue you're dealing with, it's important to know this quirk of your wheelset. I run Mavic wheels with FTS-X freehubs on both road and mountain, and I can recommend them without reservation - but you really do need to do the hub disassembly and bushing re-oiling at least once a season.