Basically it amounts to pushing grandma's version of "tradition" (however she might view it in her own head at any given time) on to the rest of the family come hell or high water.  It isn't the octopus or Nutcracker so much as it is giving Grandma the satisfaction that the only traditions worthy of adhering to are her own.

Somehow it is inconceivable to them that traditions evolve and some might actually disappear as the next generations decide how they will celebrate Christmas and other holidays.  My own in laws still operate with a very specific agenda when it comes to Christmas and take great offence when others suggest "gasp" trying something a little different or adding their own traditions when it comes to the "Traditional" meal.