This reminds me of the story idea I'd love to see done for Keine x Mokou
, where after the former dies, the latter waits for her reincarnation and so they cycle through again and again. After all, Mokou literally has all the time in the world, and having that to look forward to would keep her sane in the between times.
As to this story, I guess Reimu worked out a deal with the Yama and Yuyuko to allow their annual returns in a Dragonball Z kinda way.
As Thiron said, this is just like the obon festival, just in Incident form (since Marisa refers to it as being an incident and in Japanese you get the feeling it occurs at the same date every year).
So, every single year that occurs and the current Hakurei gives it a free pass and let's the spirits wonder about.
What caused it is unknown, but I'm just gonna go ahead and assume that MariAli is too strong to be bound by mere inconveniences such as death.