I planted a huisache tree (Acacia farnesiana) in our Houston-area back yard a couple years ago, and it has grown rapidly since then. Last week, I was surprised to find one upright of the multi-trunked tree broken, collapsed forward in calm weather, only attached on one side to its previous position. Upon inspection, I noticed that the back side of the branch was surprisingly clean-cut, with only the middle showing the jagged break that I'm used to seeing when branches break. I removed that one late last week. Then yesterday, walking by the tree, I noticed that a second, larger upright was completely girdled with a shallow cut, about a quarter inch high and extending through the bark into the wood underneath, perhaps 1/8" deep in all. Sawdust-like debris was on the ground below. The cut is at almost exactly the same height as on the first severed limb. I'm fairly certain that this damage is new since I removed the first branch (it is glaringly obvious). Any I'm not aware of any gnawing critters in our garden (besides my puppy, who doesn't have the oral dexterity to complete this feat). The backyard is fenced in, with locked gates, or I'd wonder about human interference.

ideas what can be causing this?

Severed limb at left, girdled one at right

Close-up of the damaged area