Mac Mystery

Just reaching out you mac experts. I used to think I was one, but this one has me stumped.

Using Safari, every so often, while I am doing something (what?) shrinks the fonts of the page being displayed. In other words, does the equivalent of a Command/- or Command/+. It seems to have something to do with dragging the mouse while doing something else.

I know that a command-0 restores things normally. I also know that control-double-fingered-drag will enlarge the whole screen, that is, zoom in on the pixels. This is something else.

I can’t figure it out and neither can google. Do you know?