The news conference (very well done by the spokesperson, BTW) mentioned that the shooter was scheduled to take a drug test today as part of his commercial driver's license process. The spokesperson did NOT say that this was a factor, but I wonder if it might have been. If he had done drugs recently and knew that he would fail the test, it might have triggered him to act today. But that is purely speculation and could be 100% wrong. Time will tell.
Doesn't matter... There are no "reasons" for a heinous crime such as this.
Anything is possible in these sick minds.