With the clarification in the comments that you've already confirmed the layoffs with your boss and offered to quit to save them the problem simply because it would impact you least of your coworkers, honestly, I can't think of a better "why I left" way to spin this whole thing because what you did it exactly right.
You didn't jump the gun on the rumor - you went to confirm it, and then, taking everything into account, you've offered to resign in exchange for having a letter of recommendation and interview flexibility - while still working for the company and delivering the good work during the transition period.
You really do not need to say anything else but the truth, it's really the story interviewers will want to hear as it shows integrity, sensibility and amazing amount of empathy - more so as you didn't know who will be fired and whether that will be you (unless you are in Germany or a country with similarly rigid firing order).