From some of the articles I have been reading/googling it looks like there can be multiple reasons as it can occur doing different sports or activities.

This link http://www.joe-cannon.com/rhabdomyol...ness-bootcamp/

It even goes as far as spinning can cause rhabdo. Albeit I have not tried Crossfit myself but hope to one day in the future (I like variation in my fitness), I think circumstances will cause this rare ailment that can happen across many planes