I've heard all kinds of stuff about how non neutered males can be aggressive/territorial I'm surprised it's taken them five years but that's probably a good thing!
Last week one of my gliders was getting unusually aggressive so I took both gliders out during the day and while they were sleeping I rearranged the entire cage and cleaned the bars very thouroghly (trying to remove any scent in case it was territory marked or something) and then I reintroduced the gliders to the cage later that night once I was sure the cleaning supply fumes were gone.
The gliders haven't been aggressive since then and I think it is because they think they are in new territory.
Hope this helps.