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.
Yeah, it was the exact scenario for a Danforth that's worst--the reverse pull. It's possible
there were some weeds, but the cruising guide said it was mud, and when we pulled up the anchor the next day, it was all mud. But that was a good 100 feet from where the anchor was originally set, so yes, it's possible.