More to it than denial of leave, IMO. We may find out he was also recently denied consideration for promotion. Living off base with a wife and young daughter on an E-4 salary would be a challenge for any man.
“Lopez, his wife and their young child moved to the Killeen area, where Fort Hood is located, in February. Prior to the move, the family lived at another Texas military base, authorities said. The Quail Circle apartment Lopez lived in is approximately two miles from the military post”
http://www.huffingtonpost.com/2014/04/03/who-is-ivan-lopez_n_5084315.html
He was denied leave last Fall when his mother died. He had to beg for it. They gave him 24 hours. Then they changed it to 48 hours. Then I bet they did not approve paying for his travels due to sequestration. There is more to his snapping. But it will trickle out, I am sure. And I am speculating about the paying for travel part.