On early 350 B's the dreaded DU bush would come unglued and yes you would get close to ground resonance. It was a case of recognition of the onset and carry out the correct action. 1st get back in the air, move to a different surface, place aircraft on the ground and plant it there, do not pussyfoot through to flat pitch a la Lama technique. Wait to see what happens, if the GR symptoms aren't apparent, shutdown, if not, repeat. Do NOT attempt to shutdown if the symptoms are there.
I would guess this is a "Carbide Star" fitting that has come loose. Wouldn't be apparent from the damage after the fact. This has happened plenty of times on very "new" aircraft. There is a lot of factory documentation about this. Hands up who really adheres to the BFF, TA and ALF inspections on this model aircraft? If you don't know what these inspections are and you are flying one, simple remedy, don't continue until you do!