I wonder if a ground burrowing critter is eating the roots. Or ants? I seem to be battling ants this Summer.Did you give the trunk a good yank to see if the roots are intact?
These get pretty much full sun being on the north side of the field.^^^^^^^ Your Violi's crab looks much better than ours. Ours gets morning shade, so that may be slowing it. Yours looks great!!