If my estranged wife moved out of state without my knowledge while I had phyisical possesion of the children, is this abandonment?

In actuality, it is abandonment, but it may not be considered legal abandonment... yet.

Texas is a no-fault divorce state.  Some states require a spouse to plead a valid reason for seeking a divorce.  Texas does not impose this requirement.  All that a party must allege is that there are irreconcilable differences and you are eligible for a divorce. So as a basis for divorce, her leaving the state won't help or hurt you. 

Where it does help you is in the area of who should get custody.  If you have physical possession of the children and she has left without providing any support for them, then you have a very good chance of being name the primary managing conservator of the children.  This is Texas' fancy way of saying-- you get custody. 

Depending on how long she is out of the picture, then you may have other options.  In Texas, if a parent fails to support their child for a period over a year, then their parental rights can potentially be terminated-- however, this is not automatic.  The judge still has the final say and must find that the termination is in the best interest of the child.  Many are reluctant to grant these motions for absenteeism alone-- because it can also limit the potential for child support and certain inheritance rights.

You don't necessarily have to file for divorce right now... but considering that she has left the kids and you have stepped up to take care of them, you would be in a good position to file if you could get it going while she is still out of the picture.