Simon would know more than me but my thought was freezer burn, that some moisture got into them, froze at the edges, then of course thawed out when you took it out but damaged the emulsion when the crystals formed. Are you careful to put then in a sealed container before freezing or do you leave them in the freezer in just the original packaging? Sometimes that's not enough. That's my best guess.
Mamiya: 7 II, RZ67 Pro II / Canon: 1V, AE-1, 5DmkII / Kodak: No 1 Pocket Autographic, No 1A Pocket Autographic | Sent w/ iPhone using Tapatalk