What factors aggravates ear infections post grommet falling off with the opening for the grommet still being visible on the eardrum?

The same factors: that promote all ear infections: spread of infection from preceding upper respiratory infection causing fluid build-up behind the ear drum; smaller and more level eustachian tubes in children impairing drainage of inner ear fluid. While tubes are very helpful while they are in place they usually fall out after 6-12 months, the hole heals, and infections may recur. Hope this helps:)