The one thing I wish I could do over at your age was working harder at interacting with my family, especially my stepmother.
I had a similar situation I discussed in therapy for a zillion years (took me that long to understand it :-) and my therapist pointed out that "Would you like to help?" is a conversation starter! You mother was doing her best (just because she is an adult doesn't mean she has learned how to converse with others any better than you have!) to engage you but leave you free to not want to be engaged. Maybe she has put herself out in the past trying to start a conversation with you and you have seemed to blow her off like this time? That can hurt another, even a mother/adult! Even if you had been honest and told her you thought you and your brother were supposed to be doing the tree and felt your mother had pre-empted your job, it would have been a conversation and you might have gotten much further in understanding your mother and she, you! She could have explained why she was doing it (you and your brother hadn't shown up to do it and it needed to be done, she thought you were not interested, you hadn't said anything about it) or she could have apologized and been truly sorry or could have told you she was worried about something and decorating the tree, doing something with her hands made her feel calmer or more normal; you can't know now what could have been!

Relationships take practice and being together and doing things together. Walking away just hurts you, you don't get that practice and opportunities and have to try to fit them in later, when it is harder because there aren't as many people "around" to practice with.