I've Been Wondering, What Was So Different About The Timeline Frisk Fell In That They Were Even Able

I've been wondering, what was so different about the timeline Frisk fell in that they were even able to fall? Flowey went through hundreds of resets without them, and their first appearance broke his control over the timeline instantly.

I think the reason Frisk's timeline went on longer than all the others was related to this event...

Flowey recounts this story with puzzlement and some hesitation.

Toriel remembers this event - which means it happened on Frisk's timeline.

We know this meant something to Flowey because he leaves a glass of water for her after putting her back to bed. This, when he's killed her multiple times.

Flowey may have let this timeline go on longer than all the other ones because Toriel saw her son in Flowey, without having to be told, and he still doesn't understand why, so he put off resetting the timeline long enough for Frisk to fall.