That happened to me a few years ago for the 4x4, started on a plane flight and continued though the day. As I recall I dropped 40s-1min off my average. I didn't think too much of it at the time because I still wasn't very fast. ~2m
I dropped 30 seconds off my 5x5 last fall, and started to get sub-3m times, now I'm at 2:20.
Then in the Aurora Summer Open I dropped 30 seconds off the 5x5 and got an official 1:56 average (with best of 1:44.41). I've never been able to get close to that again.
Weird things happen sometimes in cubing.