Do you love big bluegill?
I don't think they're sterile. If someone was stocking your lake with hybrids, I can see where they could out compete the bluegills. Where I catch them, they are all "nature made" hybrids and are quite scarce in number compared to the pure strain species.
I take my fair share of bluegill/green hybrids for the table. From online literature the hybrids are fertile but more then likely they are mostly males. Hybrids are also low in fiertility and mostly males offsprings. I think harvest of these hybrids is a good things.
The hybrid green gill (HBG) if in high populations could be cutting into the BG populations by eating the BG fry since their (HBG) mouths are large enough to eat older specimens of BG fry. And I suppose if the HBG are in high enough numbers natural hybridization may be taking place or even crosses of the regular female blue gills and the HBG males because of the shortage of male BGs. I am just guessing as I am a not even a novice on the subject, just a learning wannabe. I have also read that the green sunfish and their crosses tend to be harder for predators to catch and consume.
Your situation does seem to be counter intuitive.