510 Pyro at 1+100 is a very reliable concoction and the only time I've found it to give problems with unevenness is at higher dilutions like 1+300. So I think maybe it's the agitation. Usually you'd think the stripes or surge bands caused by uneven development would be across the film, especially with 35mm and in relation to the sprocket holes. But with 120 maybe the extra width of the film is allowing ripples lengthwise as the tank is inverted? I never invert the tank with 510 Pyro and just use the twiddle stick, an initial 30 seconds then 10 seconds on the minute unless I'm doing a semi-stand development. As a compensating developer gentle agitation is most appropriate as you need to give it enough time to exhaust in the highlights between agitations, so maybe you've done too much agitation for too long and it's not having time to work evenly across the negative?
