Its generally the switching, although I wouldn't professionally sign off an undersized relay on the basis of 'it never switches, so its fine'.

The switching under load causes the contacts to get worn, then the large load causes the now-worn contacts to get hot, and the plastic supports melt and catch fire.

A mains connected circuit designed to minimum costs could have all sorts of corner cuts: bad capacitors, no overvoltage protection, no back EMF from load protection, bad isolation to the driving circuit, etc, so that it could work flawlessly for decades, but also stop working after a while because of a spark that welded the contacts together, or mains voltage to enter the driving circuit because of moisture and insufficient creepage between high and low voltage tracks, etc. I use a lot of cheap Chinese products in non critical contexts, but I'd avoid them with mains voltage or in any safety critical use.