Nobody can shed any light on this. You have not provided any system info or other technical details.
Hitting the = sign on the main keyboard zooms in on the timeline if it is active, Pressing the - sign zooms out.
Holding doen Alt/Option while dragging up or down on a trackpad or using the scroll feature on a mouse will zoom in and out of the timeline if it is active.
There are lots of good things to be found in the Keyboard Shortcuts Reference: After Effects Help | Keyboard shortcuts reference. Check them out.
Happens to me regularly when using the Apple wireless mouse.
As I said, Mylenium, I use multiple systems and configurations - the same issue happens regardlessly. It is not a system issue.
Cheers for the suggestion Rick - I initially assumed it was a keyboard shortcut that I was hitting accidentally. However, the shortcuts for zooming on the timeline go by much higher increments than 15%. It's as if I've dragged the slider just a tiny bit.
Bogiesan, I do use the Apple mouse on one of my machines, but the same thing happens on my PC as well.