The problem is usually that the ink on the backing paper interferes with the emulsion on the turn of film above it. On very old (and very cheap) film the ink can even glue itself to the film. Old Ansco film with black paper and white ink was very prone to the problem.
If the 'o o o 1' print-through sequence is centered on the first negative frame then it is printing through the back. If the first frame picks up 'o o o 2' then it is picking up ink from the frame 'below'. If frame two shows 'o o o 1' then it is picking up ink after it was wound on.