I recently got some XP2 Super processed (rolls of 120 and 35mm) and on both types, I got a load of crud deposited on the negative to the point that I've given up hope of ever being able to use most of them in any meaningful way.
My cameras don't have bellows and I'm assuming that the detritus is linked to the solutions used for developing - possibly (probably?) the result of old / over-used chemicals that contain goo that wouldn't be there if the machinery was cleaned more regularly.
If you dev'd your own, I'm assuming there's some issue with the film. Like you say, it seems unlikely that anything from the camera would be transferred and become part of the film and wouldn't wash off when rinsing.