I don't think it's so much a[n apparent] drop in Mavericks usage as it is in OS X usage. According to my calculations, OS X 10.10's share of OS X users measured by Net Market Share actually went up from 56.2% to 58.8% between May and June.
As for the overall figures, in the few years that I've been seeing the Net Market Share numbers, this kind of retrograde motion hasn't been uncommon.
