Honestly... no, I hadn't given that any thought. However, it only makes sense to consider both parents when looking for an answer!
The stallion in question is none other than our very own boy, Indy Mood. This happened the first season that we owned him, although we had bred to him in the past (to other mares, not this one... this was a maiden mare) and hadn't come across this before.
However, he is a VERY FERTILE boy... I'll spare you the details, but he excells in every aspect of his job!
He always gets an A+++ for settling his mares, and his fertility is awesome, but I had never considered that this might lead to an increased probability of twins. However, none of our other mares have had this problem when bred to him, and this particular mare had it happen twice in one season.
I don't know... sure gives one a lot to think about.