V.W.
I'd suspect some of it is the fear that it won't "feel" like Christmas to her. Like another poster on the board today mentioned not yet feeling the Christmas spirit.... she may be worried that she isn't either and won't on the 16th. :/
And, having it early, may have her stressed/worried that she has forgotten something. I mean, I usually have most of my shopping done with about 10 days left to spare before Christmas, but I almost ALWAYS find a few things I have stashed away that didn't get wrapped yet, on Christmas Eve when everyone else is already gone to bed... So maybe she is trying to keep it all straight in that regard and thinking she won't get all the "put away" things out of hiding and get them wrapped. Or maybe she hasn't come up with the "right" gifts for everyone yet. That is always tough... when you are hoping that a super idea for someone will appear out of thin air, and time is dwindling for it to happen.
But probably, it is mostly stress about how things will go and whether she will feel like Christmas or whether she will feel like it wasn't "really" Christmas... and then on Christmas Day she'll feel let down.
Not at all suggesting that you shouldn't do this, only posing a theory about why she might be feeling stressed. Any sort of change can stress people out. Changing an event with as much emotional baggage as Christmas memories is just going to create the opportunity for the stress to be a bigger factor. But, it is something she will have to deal with, and it sounds like she is. Next year it will be even easier. :)