I’ve seen that before. Developer not reaching the film was the culprit. Likely the film was “dented in” on the developing reel letting adjacent layers touch each other.

Reminds me of the quote:

“When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much he had learned in seven years.”