Once you have experienced blowing up a reversed elcap you will never forget its orientation. I never understood though what makes it leak current and hence heat up.
Modern electrolytic caps don't burn like they used to.
The last few times I made a mistake, there wasn't even an explosion, even less a short-circuit. The thing slowly boiled and bubbled or unfolded.
Anyway, it blows up because the capacitor's insulation layer isn't some stable material, it's a tiny oxide layer built over the metal plate by anodization. If you put a high voltage on it with the wrong polarity, you reverse that anodization and short the liquid and the metal electrodes.
There's an aluminum oxide layer as a coating on both the anode and cathode inside the (electrolytic) capacitor. Under forward voltage it will gradually thicken but under reverse voltage it dissolves and causes a short. This increases the temperature which causes hydrogen ions to separate and bubble through the material, increasing pressure within the capacitor package until it bursts.