I think there is some confusion in terminology. Steel case ammo does not cause fires. Steel core ammo (the bullet has steel inside it) not only can make dents and divots in the backstop, it can make sparks. If you see lots of sparks coming off the backstop, stop and check your ammo to be sure you are not shooting steel core ammo.
When a cartridge fires, the powder burns and spits the bullet out the barrel. Well, most of the powder burns. An unburned grain or two of gunpowder often goes downrange and lands on the floor. After tens of thousands of rounds have been fired, there may be enough grains of unburned powder on the floor to catch fire if lit off by a spark.
For that reason, range officers have little magnets to test if a bullet has steel inside it. (Lead and copper are not magnetic.)