What about the algorithm changing over time to favor longer posts and content creators on the platform adapting to the change? I suspect you’d see the same pattern with the average length of popular non-music YouTube videos over time.

Good point, that's a good explanation. I think the timing with ChatGPT and how consistent it was for 5+ years before that make for very strong circumstantial evidence, but you're right that there is at least one other good possibility.