The most likely cause is some form of mechanical damage to the film. Abrasion from rolling the film too tight, or folding marks where the film has kinked while loading the developing reel, are the two commonest culprits. If the marks are crescent shaped, then it's almost cetainly caused by the film being kinked.(posted 8829 days ago)
Film is delicate stuff, especially 120 size, and you need to take a lot of care in handling it.