That looks like a rather flat trapezoid for something that fell from high above.
With a fast-moving object, we can usually tell its trajectory across the map much more accurately than we can tell where along that trajectory it impacted the ground. See: MH370.
Maybe fits the "DoD is shooting something at some kind of incoming drone" explanation - they know they're shooting _from_ the top of the trapezoid but in terms of direction, only that they're vaguely facing south. (Doesn't really explain why the TFR doesn't extend into Mexico though.)
The area they would expect to find it would be much narrower than the area they would expect a plane overhead to be able to observe it.