Help! Any idea what is causing this?

Have had mixed success with FP4+ 120 film using Rodinal 1+100 for 1 hour semi-stand. These marks appear from time to time in frames 9 and 10. Using Paterson tanks, I develop by placing chemistry in tank first, and then doing anti-halation soak in a separate tank. Therefore, am not pouring chemistry into tank, but rather dipping reels into chemistry. Only agitation is 15 seconds at beginning, and 4-6 very gentle agitation at 30 minutes. This went away for a while when I modified to above sequence (vs pouring into tank).

Any thoughts/comments as to cause or preventative ideas would be greatly appreciate.