Because everybody knows that non ferrous metals (Lead and copper) ignite things just by looking at them.
Tracers?!?!
Absurd.
Not completely: kinetic energy easily translates to heat, enough heat to ignite things. Coupled with easily ignited gunpowder, it would be close to certainty.
I handed my FAL loaded with ball ammo to a staff sergeant at Quantico during a fam shoot and he fired a couple of rounds at the edge of our cleared area - it it started a fire in the trees that stopped our shooting and brought the base fire department.
Bi-metal bullets are loaded in some Com-bloc ammo. Copper wash over steel jacket, over lead core. Readily available at Walmart.
Might of hit a bag of those popper things that explode when you drop them on the ground. That might have started off the other stuff.