Roaches you find on their backs will probably be there because they've been killed by the neurotoxins used by your pest control company. Sometimes, a roach is just unlucky and falls upside down, unable to get up.
That's a good question. Generally when you find a roach lying on its back, its because its been treated by the chemicals used by your pest control company. Those products are actually neurotoxins where they affect the nervous system of the insect, which in dying causes them to flop around and end up on their back. However, sometimes a healthy roach can find itself on its back and if it can't right itself, it will just die because it can hide and it will starve.