Originally, judging by his website at the time of his release, he left because of issues revolving around the firing of his girlfriend (and soon-to-be mother of his first child) Kristal Marshall. Apparently he thought there was racism behind her firing - that's where the Michael Hayes murmurs come into it.
Now though, he's been saying on his website, and in a recent interview with Chris Cash he says he left because WWE told him they have nothing for him at that moment - the fact that he was being pushed to the moon at the time of his injury makes that unlikely imo.
He;s currently working occassional dates for Booker T's fed in Texas I believe. Either holding out for a TNA deal.
Interesting that there's rumours about American Street Fight - shows that Lashley is NOT a big draw anywhere - he was probably hoping for a UFC deal like Brock got, but he was never as big as Brock Lesnar, never will be. (In name terms as well as physique

)
EDIT: Osama - Looks like your sources were correct:
http://www.mmaweekly.com/absolutenm/templates/dailynews.asp?articleid=6381&zoneid=13
Guess he's gonna work his way up...