What caused this?

Unfortunately It looks like she damped off... It’s a disease that infects seedlings. I highly doubt it had anything to do with your lighting schedule or intensity... That wouldn’t cause the pinch and die effect, lol. Consider applying a microbial inoculant to your seeds to prevent this. Also take a look at this article from Purdue, one of the worlds leading universities for agriculture. It’s almost identical to your photos. https://vegcropshotline.org/article/damping-off-of-vegetables/

Hard to say exactly without a diary or much more info, but they usually fall over when there is not enough light, not too much. For 9 days I would expect more growth, this could also point to not enough light. By not enough light, I mean strength, not day length. 18/6 is optimum. Cheers, Organoman.

You want to be on 18/6 for your light and if your plants are bending over to shield away from the light then it's to close.