To get back to the ops original question...
Everyone says it's absolutely mandatory to keep the SAO with safety on. Now all things being equal, if something gets in the trigger guard and more than 5.5lb of force is put on it, it'll fire.
I guess what I'm asking, why is safety use on SAO pistols mandatory? It could just as easily happen on the other trigger types.
Ok let's look at some real world guns. The Browning HP is a single action pistol. If you are carrying the gun in hand or in a holster without the external safety in place and the hammer at full cock, all it takes for the gun to fire is for the trigger to be fully depressed. So if the safety is not on and "something gets in the trigger guard" and fully depresses the trigger it will go off.
On the single action 1911 without the safety on and the hammer fully cocked it requires that the trigger be fully pulled and the grip safety fully depressed for the gun to fire. Same with the XD.
A common ND with the Glock is that folks shoot themselves while reholstering because either the finger or part of the holster "gets in the trigger guard".
So as folks here have said best to keep things out of the trigger guard.
But it is an extra margin of safety that makes it wise to use the external safety on a gun that has one while carrying it and disengage it when you are ready to shoot. The point of a single action gun is to provide the advantage of a short and light trigger pull. This comes usually at the expense of having an external safety of some sort. So carrying a cocked single action pistol (or a da/sa gun cocked) about without the safety engaged is like carrying a cocked revolver, not a good idea.