Loose wheel nuts
until Petite i have not run an buggy but i have been running the touring cars with same M4 nuts.
I never had a Kyosho nut come loose ever until we started using new Nylon Sweep wheels. Every body at my club had wheels flying off during racing. My wheels did undo but never flew off. We just changed to locking nuts and the problem was cured, or you could do as roger mentioned use some threadlock.