U-869 was an enigma for a long time. Little was known about her last patrol and among the few facts "known" was that both U-boat Command and Allied Forces thought that she had been ordered to an operational area near Gibraltar. However, as we know, the boat was found off the New Jersey coast, leaving post-war authorities in great doubt as to how she was sunk there. Upon investigation, it was discovered that (although there was no indication that there was a U-boat in the vicinity), the US destroyer escorts USS Howard D. Crow and USS Koiner both gained sonar contacts on Feb 11, 1945 in the location where U-869 was later discovered. The "Crow", manned by a US Coast Guard crew, dropped depth charges on the contact and found some sign that they had hit something but not enough evidence to be certain. The "Koiner" obtained a sonar contact (slow moving or stationary) shortly thereafter in the same area and dropped depth charges. Again, there was an amount of oil and wreckage but not enough to completely confirm a submarine kill. Based upon the location reported by the DDE's, it was considered that they'd likely been involved in the sinking of the U-869. There are theories that the first attack may have sunk the U-869 and the second an attack on her already sunken hulk; or perhaps the first attack caused her to "hide" on the ocean floor and the second attack was fatal; or maybe the first attack damaged her and the second attack finished her off. The damage described is very much like the explosion of a weapon in contact with or very near U-869's hull but it is just as likely that a depth charge -- either dropped on the operational boat or it's already sunken hulk on the ocean floor -- did the damage described as a "circle runner" torpedo.
The DDE's were escorting a small convoy leaving the New York harbor area. It could be argued that U-869 was sunk by it's own torpedo and *both* the depth charge attacks were on its already sunken hulk, but the convoy was small and the escorts were able to observe all the area around it. No unexplained explosion was noted at the time (and a circle runner explosion would have been near the surface and thus would have created a large "plume" and noticeable noise). Also, the contact during the first attack appeared to be moving.
It is impossible to know exactly what happened and its sequence (the only people to know have been dead for over 65 years) but based on the evidence, it appears that an attack by one or both the DDE's was responsible for U-869's sinking.