To be clear ... it appears that he's being fired for a failure to report his whereabouts, not for responding to the "officer down" call. While I'm no LEO, my guess is that it is SOP to report when you are leaving your post (I know it was for us in the Army). In fact, in the Army, you could be subject to a court martial for leaving your post.
A simple radio call to his shift supervisor advising that he was leaving his post to respond to the "officer down" call would have saved everyone a lot of angst it seems.
This statement is telling:
Kevin Lawrence of the Texas Municipal Police Association agreed. "You don't fire a guy for this unless he's a chronic disciplinary problem," Lawrence said. "You call him in, you counsel him and you put him back out there. If he's a good cop, he's a good employee. You use this as a training opportunity."
I'm guessing that there's more to the story than was reported.
