I call any small explosives intended for anti-personnel purposes with enough powder to incapacitate and/or maim "toe poppers", instead of the actual u.s m14 landmine. It comes from military vernacular.
the explosive device was most likely planted under a certain amount of dirt with the mechanism to fire it attached to the flag so that if it was pulled, or kicked in this case, it would trigger it. You'd want enough dirt on top of it so it doesn't also get yoinked out with the flag without going boom, so it'll have a bit of topsoil on it to keep it tucked way.