The most common cause is what is called shoot-through. And it gets a bit complicated but I’ll try…
FETs are little switches. But they have something called a propagation delay, which is a short time period where they are half-on and half-off. This is where a lot of the heat comes from too… during this time they get a little hotter than usual due to the extra power dissipation.
For this reason, when you have FETs installed in an opposing fashion (so that one side drives the polarity this way and the other side drives it the other way), you must be sure that both sides are not on simultaneously.
Because of the propagation delay, that means you need a bit of dead time in the middle where both sides are off.
If the dead time is a little bit too short, you get into a situation where both sides are inside that partial conduction zone, and you have the full battery current running through both. Over time that can blow up a FET. This condition is then known as shoot-through. It tends to essentially “weld” the FET in the closed position, causing a dead short over the DC bus, and setting the house on fire… or at least blowing a fuse.
The reason could be as simple as a FET that was not quite up to spec, that had a slightly longer propagation delay than it should have had. In other words, it is not necessarily a design flaw, and it is not necessarily anything specific (like a spike, surge, lightening, etc). Sometimes a FET just fails. If it happens too often, the engineers will tweak the design, use a more expensive FET or increase the dead time, etc…
Also see here: https://www.youtube.com/watch?v=UPfUn5ki7OM