One anomaly brought to Sunset to Gravity Falls, she believed that in Gravity Falls would not bring any problems from Equestria, but she did not know that the anomaly duplicated some things from Equestria that are now in Gravity Falls.

If the lines of time created by Starlight Glimmer before they disappeared some of its inhabitants ended up in another world, more specifically in a town called Gravity Falls.

What would happen during the wedding of Shining Armor and Princess Cadance, Twilight had been taken to Gravity Falls and her friends could only watch without being able to do anything.
It is a parallel story to https://www.fimfiction.net/story/499451/the-power-of-the-gravity-falls

An entity begins to transport inhabitants of Equestria to the world of Gravity Falls.