Is this the same gauge you've been using all along? if you have gone up in gauge size, the string might be binding in the slot (if that's where it's breaking).
Is the bridge or nut damaged in anyway? It might not be something you noticed happening at the time. Did it shear off, perhaps leaving a sharp edge?
Do all the other string manage ok? I would expect the e & B strings to manage, but how about the other wounds?
...so ever since then, I always hang on to the buckle.
