It isn't the voltage but the current flow that will start a fire. You'd have trouble starting a fire with 12 volts of AA cell batteries but a 12-volt car battery would do it.

That's a lot of electrical underground. Was it all installed by qualified electricians who were aware of all the other electric in the area?

All of the electrical in that area was installed by the same person about six years ago--a licensed landscaper. It was installed in a bare back yard (a brand new house), so there shouldn't have been anything hidden. The current was on a few hours before--the lights turn off automatically at midnight. Perhaps the mulch had been smoldering for a few hours?

I do wonder if the electrical wire to the light was weakened--maybe hit with a shovel (by me) at some point or gnawed on by a gopher (I've had them).

All of the electrical in that area was installed by the same person about six years ago--a licensed landscaper. It was installed in a bare back yard (a brand new house), so there shouldn't have been anything hidden. The current was on a few hours before--the lights turn off automatically at midnight. Perhaps the mulch had been smoldering for a few hours?

I do wonder if the electrical wire to the light was weakened--maybe hit with a shovel (by me) at some point or gnawed on by a gopher (I've had them).

Even with the low voltage there is a possibility, very low probability, of a fire starting from a short. If the mulch is thick enough and is doing a good job of holding moisture it should be extremely difficult for a fire to start, and if the lights were off at the time there would be no voltage in the wires to each lamp, unless each lamp has its own off switch. There are reports that mulches of 4 to 6 inch thickness have spontaneously combusted, something I've not seen yet, usually it is a much larger pile of organic matter something like a compost pile.
I have seen wood chips used as mulch burn when layed down not thick (4 to 6 inches) enough because they dry out, there is no moisture held in those chips and someone tosses a cigarette butt into the mulch and it can ignite from that source.

One time years ago I was driving up rt95 in Freeport, Maine and noticed a state highway crew mulching the center median.
The next day on the way back south by the same area traffic was backed up for miles because the median was on fire...
I don't know if it was because somebody lit it on fire on purpose or if it spontaneously combusted but it sure smoked alot.

It doesn't take much electric current to start a fire. The best bet would be to have that checked out. I have seen landscape mulch on fire in a parking median before (cause unknown) and it was actually kind of hard to put that fire out. Use of various power tools and vehicles could theoretically make a spark sufficient enough to start a fire as could a discarded cigar, etc. One of the problems that people discovered when railroads started going past farm land was fields being ignited by sparks from the passing trains.

Bob64 is right, take 2 D cell batteries and pull apart a piece of steel wool. Hold the two batteries together + to - and bridge the far ends with the steel wool. Be careful its easy to get burned.

Its a heckofa trick for firestarting in front of kids if you put the steel wool in the fire and wire it remotely, like magic...

I've used that, or old disposable flashbulbs, as igniters in tripwires (usually to set off firecrackers).

I too was called by my alarm company. It seems that my mail carrier saw my mulch on fire in front of my home, caused by low voltage landscaping lights. The wire on one of the lights is completely burned (only bare copper wire left) How could low voltage fixture have caused this?? Could the transformer have stopped converting line voltage to low voltage. The system is 3 years old.??