I was told it was to do with stress. I first got a twitching eye the day after I'd completed my second year at university, and had it for nearly a week after. A tiny but very frustrating movement that made me wince, and one I could barely see even when focusing in the mirror. I got it again shortly after my third year exams had finished too, and for a day once or twice in my fourth and final year. I don't have it now but believe being reminded about it, including in typing this post, might kick it off again somehow. Not superstitiously though, I genuinely believe thinking about it more reaffirms it.
Although the times when I got it would theoretically be the points in my life of least stress (no more exams, mates still around, can go nuts with them), perhaps I still had stress from the occurrence due to the end of one routine and the urge to find a summer one that didn't really come, as well as having to move about so often.