You may have been over a weed bed and in which case the anchor was not holding. You said it is a mud bottom, but are you sure you were not in some weeds.
Also, the Danforth is probably one of the worst designs when it comes to resetting. IIRC.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White