I don't remember what sort of dwelling Key lived in before. Was it different? I didn't particularly do that on purpose so we might be able to just pretend nothing changed.
I imagine the breakup was just as awkward as their relationship, but a mutual agreement. Since they were both still fond of each other, it probably hurt some, but they tried to leave on friendly terms.

Any of those reasons sounds good to me. They might seem to butt in on Key and Priscilla's new relationship and make jealousy and create tension. I'm more fond of 2 or 3 so whichever sounds more reasonable for Lorn to do.