listen guys: we all know what flygon is based on, right?
its the dragonfly. dragonfly-flygon almost and flagon is dragon-type so that makes up for the lack of "dragon" in flygons name.
Flygon is based on the winged, adult stage of the antlion, which highly resembles a dragonfly
well, the antilion is a bug too, so that makes sense? say yes
look at this too:
some species of dragonflies are commonly called "sand dragons",
yup. sand dragons. flygon is ground too, so it seems like that makes even more sense about its bug egg group. if we look at flagons earlier evolution stages, notice the first one called trapinch. its ability is called sandtrap and antilions makes small holes in the ground which they trap other ants with and eat them. very similar to sand trap. trapinch does also look very similar to ants, and although antlions and ants has different families, some of the antlions looks somewhat similar to the ants and trapinch
so thats why flygon is in the bug egg group.
hope I helped :D
source: this and pure logic