I think the pressure and schedule got to him. Jeff doesn't seem like he can handle too much stress. Plus he already had two wellness policy violations, if I recall correctly (and that's not counting his run before) so that probably didn't help things. I wouldn't be surprised to see him back someday though, he got insanely over during his final run.
He couldn't handle the strenuous schedule that WWE is known for, and he had injures and other problems affecting him.
More predominantly though, he had drug problems and was failing a lot of wellness policy tests, he went to TNA where he could use those same drugs and not get in trouble for it. Pathetic really, I have no respect for the druggie, spot monkey.