Pretty sure the ridiculous sequence of events in the accident was Yuuka snipping together a thousand different things to make it happen.
But why did Yuuka decide to use that day of all days to make Ayaka run to Yuuka and finally piece together that Yuuka is a witch? She could’ve waited till she was 17 or she could’ve done it earlier. Only now that she’s in her teens and finding romance, she chose to blatantly make the worst case scenario happen with various small stitches for the most shocking reveal.
I’m assuming that when Ayaka and Michiru kissed, maybe the next day A, Ayaka had told Yuuka and Yuuka was able to edit away the kiss, and edit away the memory of the next day-A so A happened again without her knowing? I think the snipping Yuuka does really does in fact change Ayaka’s “memory” about days that happened, such as a day happening 20 times but Yuuka can strip away 15 of them if she wants.