Or buggy software trimmed the plane all by itself.
That is possible also, but I would think much less that a minute’s elapsed time for the change. Or maybe not, automated control systems don’t get tired like humans and need the trim to reduce the force required to maintain a selected control input. I used to instruct instrument flying and occasionally had to “train” a student to use a light touch by setting very high, unusual trim tab inputs and have the student fly straight & level, maintaining altitude and heading until his arms and legs began shaking...then I allowed him to trim the forces out. Most weren’t “ham-fisted” after that and became much better instrument pilots.
JC