35mm, right?

These dark areas are from less development, in the area just next to the clear unexposed margin between frames. Doesn't make sense to me that it could be surge or agitation patterns.
Maybe could have been a dark thing in front of the camera when shooting, maybe in motion, but it doesn't look like that to me.

I can't think of anything else, unless some kind of physical disturbance to the film before developing. ??