The latest version of Opera is weirdly, incredibly sensitive to stray clicks on the border of the vertical scrollbar. When clicking fast on the bar area or the down arrow -- which is pretty common during browsing -- the slightest nudge of the cursor towards the border area shoots the window into "restore" (small) size.
Have to re-maximise it, start scrolling again, and then blam! it's small again.
This has happened to me something like 20 to 50 times today. It never happened in previous versions, and it's never happened in other programs. I doubt my browsing behaviour has changed overnight.