It's the grenades these dumbasses "left behind" on the order of king dumbass at the start of the chapter. Given the average IQ in this manga seems to hover around the lower end of the spectrum, I suspect they just left all their grenades in a big pile outside the cave with no one to guard them, because all 50 soldiers had to squeeze in. You know, as you do. With nobody outside to call for help, watch their backs, secure their ropes, or anything of the sort. Nope. Just leave all your grenades in a big convenient pile and squeeze into the cave at the same time.