Some of the possible causes suggested here are rather esoteric. The most likely explanation is that you tightened the string too much. A lot of beginning students do that and break strings.

You may also have used too much peg dope. It should be used sparingly. If the peg doesn't turn smoothly, you may use an inordinate amount of force to tune it, and that would make breaking the string more probable.

I'm glad you took the fiddle to a luthier before breaking any more strings. I highly recommend fine tuners, especially for the E string.