Yeah, it's easy to forget *. First toolbox I saw topple was decades ago, when stacked boxes were starting to come into widespread use in places I worked. I've avoided that by never buying a bottom cabinet, although several times I nearly did (and might still do). Even the top cab on its own can show a tendency to tip. When I have them in a van, they're tied to the structure, so it's never been a problem there.
* Safety lesson forgotten - I got a shirt sleeve caught by an engine pulley, years after the lesson was drummed into me not to do that.