There are a few issues involved here.
1. The Queen has just delivered a Christmas Message in which she reminded us of the pledge of lifetime duty to her subjects which she made 60 years ago.
2. As part of that duty over the years, the Queen has made a habit of walkabouts among her subjects and receiving bunches of flowers from them. Now, I think that receiving of bunches of flowers businiess was a strange habit to encourage, because no secret is made of the fact she doesn't keep them, but it is now traditional for people to give them to her and for children, particularly, to do so, no doubt encouraged by their parents.
3. When the Queen does the same thing at the same place year in, year out, it's not unreasonable for her subjects to expect her to keep doing that thing.
4. The Queen is getting on and it's reasonable for her to change her habits and with her advancing age it's likely that there will be sudden changes to her plans due to her suddenly becoming tired or not feeling well, etc., but when one of these traditions involving the all-important connection between monarch and subjects is to be varied, the public needs to be given notice of it or, where notice is not possible, at least be given a good explanation because often they will have spent a lot of time in uncomfortable conditions getting ready for and travelling to and waiting at the venue. The statement that Her Majesty didn't want to be late for lunch, whether true or not, is NOT a good explanation, and her PR people should have been better prepared.