How did I disable the Next button in this old story?

I'm wondering if anybody can help me figure this out.

I have some modules I made perhaps 2 years ago.

Module is a series of slides, each containing a video. The next button is disabled until the learner watches the full video. When the video ends, the next button returns to a state of Normal.

This is the desired behavior and it appears to be working just fine.

What I'm puzzled about is that when I recently opened my old .story file, I see no triggers on the slides to govern this behavior (see attached screen snippet). So how did I manage it?! 

I went looking at these old files as I recently made a new module of a similar style. I put in triggers to disable the next button at start of timeline and return to a state of normal when the timeline ends, but it is acting a bit glitchy. Sometimes the Next button doesn't release - I've assigned this module to ~300 users and maybe 20 of them came back to me with this problem. This was never a reported problem with the old modules....but what did I do?! Have I forgotten something totally obvious?