Cockroaches have bulky bodies that are composed of three heavy segments supported by only six long, thin legs.
Their long legs give them a high center of gravity, meaning they carry most of their weight around their backs. At the time of death cockroaches lose muscle control, causing their leg muscles to contract and tuck underneath their bodies. Without anything to hold them to the ground, their top-heavy bodies topple over and they die belly up, on their backs. If the cockroaches that have been killed with insecticide die on their backs, this is because insecticide are neurotoxins.
The poisons that can trigger tremors and muscle spasms, eventually causing the cockroach to flip on its back. A healthy cockroach can easily right itself, but the tremors, lack of muscle coordination and, again, the rounded back and high center of gravity cause the intoxicated cockroach to get stuck that way