Any stress due to competition from the understory plants would be a terciaty concern.
The picture is kinda small, so it's hard to tell, but I don't see any dead leaves that are hanging on. which we often see with DED flagging late in the season.
If you had fenceline trees that were in a wet area that dried uo, this could be the cause of the loss, vs DED. I had quite a few trees I looked at in early fall that were dropping thier leaves early. Most were in areas that stayed wet most of the year becuase the noighborhood draind into where the trees were growning.
Some died, others showed green buds. It's a wait and see.
Piling snow on the lip of the bluff may help increase available water in spring.
If the tree does live the year, could you set up a pump so that you can irrigate this tree the next time a drought comes through?