6. You Can Go Home For Christmas, Unless The Plot Demands You Cant
What caused it: The book.
One of the standout sequences of the first Harry Potter was the Christmas segment. With most of the students gone we were free to not only to have some development between Harry and Ron, but further explore his parents with the Mirror of Erised. Had Harry joined Hogwarts four years later he'd have found that much harder to do; the school would have been full of students.
Whereas The Philosopher's Stone set up most children go home for Christmas (reaffirmed by Order Of The Phoenix), in Goblet Of Fire they all had to stay for the Yule Ball, a celebration that happens on Christmas Eve. This is admittedly a rare occurrence - it was only due to the Triwizard Tournament - but why would a school that followed a logical holiday policy so far suddenly throw it out the window.
I think when coming up with the Yule Ball, Rowling placed it at Christmas to make the whole event more special and romantic, but completely forgot her previous term rules. With such erratic term times its not unsurprising the Ministry of Magic wanted someone to come in and sort it out.