why r there so many wasps flying around my grass?

they could be a nest of ground wasps they build a nest underground with a opening about the size of a thumb. be careful they are very aggressive

True, there are underground nests of hornets. Usually, you'll see a large number of wasps all going into a single area or opening.

Another explanation is the wasps may be hunting caterpillars to capture to feed to larvae in the nest. Are caterpillars crawling in the grass (migrating) to find a place to pupate or find new food?

Wasps are predators on caterpillars and other insects. Although there is some risk of being stung, it is best to allow the wasps to do their job in nature. If someone in your family is allergic then destroying the nests may be necessary.

You probably have a ground dwelling wasp and there's a nest. If you can find the nest (carefully!!) you should eliminate it....after dark) before you accidently wake up the colony and get stung badly.