Am I obligated to continue to work on a paper I contributed significantly to? As an undergraduate, I did some group research a little over a year ago which is now being compiled into a paper. My advisor at the time is contacting us to request feedback and help writing it. I am especially important to the effort, because I am the only one who knows some of the results (and proofs) well enough to be able to explain them and structure the paper around them.

However, I am just beginning a PhD, in a completely different field than my undergraduate research. This comes with a new set of challenges to focus on, including that I have to find an advisor here within my first year. I would like to be primarily focused on making the most of my PhD program and thinking about current research, rather than having to worry too much about this old research I did as an undergraduate, which I do not feel will impact my career. Even if our research gets published (which it might not), because it's not in my current field it does not seem that it should be my top priority.

In summary, I do not think that assisting with the writing of the paper is in my personal best interest. However, I also understand that I have some obligation to the research group I was in and the research we did, and that it could be rude and irresponsible, even unethical, to drop out of the picture entirely. What exactly are my obligations to writing this paper? What is the most ethical thing to do in this situation, which also preserves my interests as much as possible?

Finally, if I should avoid working too much on this paper, how do I communicate this to my undergraduate advisor and to my fellow researchers? It would be rude to cut back on the work without any explanation.
I think the Golden Rule applies here. While you have no obligation to act against your own self-interest here, if you were in your former collaborators' position, you would probably be rather unhappy at being blocked from publication because one co-author is no longer available.

Is your preference to not work on this paper at all, or just to limit the amount of time you spend on it? From your collaborators' point of view, a few hours of your time might be  sufficient to avoid blocking the publication, while not placing much of a burden on you. It's perfectly OK to tell your collaborators, "I'd like to help, but I have many other demands on my time right now. I can devote 4 hours this Sunday to helping you get this publication ready, please let me know how I can use this time in a way that is most helpful to you."

On the other hand, if a few hours is really unmanageable to you right now, just tell them this, and let them know if you expect to have a little more time in the near future.

I suggest that you learn from this experience going forward, and make sure to document your work in sufficient detail for others to follow before you leave any research project.