A path to darkness is never something that happens overnight - it is an event that culminates over time, an event that comes from the result of bad experiences and the folly of youth. Or at least, that was the case with Starlight Glimmer.
Starlight Glimmer has never told the whole story of her life - Sunburst moving away may have been the start of her descent, but there were many more steps she took along the way.
And Trixie is curious as to what exactly happened.
GM's note: I know this idea has been done before, but I wanted to do it myself. See what I could do with it. I also know the recent episode completely removed the possibility of this being canon, but I'm keeping it up.