Theory: G5 is semi-post apocolyptic.

[Bright Honor 2,702]

So as we know G5 takes place a very long time after G4. We know form G4 that when things between ponies get too bad, the windigos come.

 

So what if during twilight's rule...something happened that broke trust between the races, causing the windigos to attack and "reset" everything?

[Truffles 2,033]

Could be, but I'm beginning to believe the movie will intentionally skip over the trigger that caused the pony races to split up and let the audience come up with that on their own. Boulder/Hasbro seems to be trying to have their cake and eat it too by using G4 lore as a backdrop for G5 but saying little if anything on the fate of the Mane 6, Spike, and the other princesses and whether they failed or if they all died before everything went wrong.