Trisolar syzygy and it's effects on the planet
Is it just for the spoiler? 

My guess is that the 3 suns aligned create a sufficient gravity field for the outermost parts - less well attached - of the planet (atmosphere, people, buildings) to be in the Roche Limit of the system, causing them to be sucked out spiraling inwards.

Here is a funny vid I found to illustrate:  https://www.youtube.com/watch?v=6e5yyIcq40o .

Hope this helps.