I'm sure it's a processing error.
Bubbles, leaving their mark... nothing else does that.
As I said QG, what amazes me is why one roll has no marks whatsoever and the other one has plenty of them. But I agree with that one in the top of another can be the problem. I ussually use 2ml of photo flo for each liter. But, again, it can pe a processor error. Well, I´ll send them to Ilford anyway. Just to be sure what is going on.