It was done immediately after Unity tried to change their license terms to increase their revenue share and introduce per-user installation fees, and before they rolled back the changes. That was quite a short window and I would be shocked if this dev flipped to a new engine in the middle of it coincidentally.As far as I remember the engine switch was for versatility to make the game as good as possible. I don't know exactly if the engine is what's causing the crashes, though.
It's also unlikely to be the engine itself causing the issues but rather the porting of a partially complete project to a new engine. Just updating engine versions can often throw up some unexpected issues, switching to a different engine entirely is pure nightmare fuel.