As for the reasons why, the main theory goes that it has something to do with Christmas fast approaching. Could it be that it's the perfect date that's not too close to Christmas for it to be cruel, and far enough away that you don't have to give them a present? Plus you wouldn't have to do the awkward family-introducing.
In fact, Christmas day was one of the lowest days for breakups.
The other highest time of the year for breakups was in late March - what is it about breakups just before the holidays? Perhaps it's the time to confront where you want your life to be going.
Who knew there was actually a science behind when you break up?