There are any number of articles about this online and the consensus seems to be it is a moisture issue. Lava rock is very porous and will absorb and retain water in all those little nooks and crannies indefinitely. Or until the heat of the gas flame turns that water to steam and the rocks explode and go flying.

You say you 'let them dry'....what exacty was involved with this drying? Just letting them set until they looked dry was probably insufficient. You might need to manually dry them in the oven (at low heat). Or one source suggested turning on the gas to a very low flame to let the water slowly but fully evaporate out of the rocks without triggering an explosive response.

Or you can replace the lava rocks with fire glass, which is completely nonporous, tempered to withstand high heat and has a much higher degree of heat reflectivity.

Same as above. We did a firepit for a client last year (Firegear Outdoors) and used their specific lava rock and lava stones (look like river rocks but made of lava rock), and had the same problem. GG and NHBabs nailed it.

So what is happening is that when lava rocks cool they tend to trap bubbles of gas inside them. when heated the gas expands and at a certain pressure the rock will explode has nothing to do with water or moisture lava rocks are no the best fill use rock or glass that has no voids

@Coldwater In your face

Heated water becomes steam which is a gas that takes up much more space that the moisture that is is when cool. So yes, moisture plays an important part in porous rocks exploding.