The problem with the Mavs is they have no poise. They will jump around, high five each other, high five the crowd, Terry will do the jet thing, and act as if they won a 'ship for a dunk that gives them a 1 pt lead in the 1st quarter.
If the Spurs acted this way Pop would throw a fit and tell them they haven't ing won anything yet.
But as high as the Mavs are for that little lead early in the game, they will be just as low and start falling apart as a team for a deficit late in the game. Thus the "choking" label.