I reckon it's Ken's appointment and relative success that has partially led to this recent trend (his age, more so than his coaching experience).
No doubt. Before Ken we had the "favourite son" phase of head coaches with Buckley, Hird, Voss and even Primus all getting gigs soon after retirement. All recent captains of their respective clubs weirdly, it was a bit incestuous and the lack of experience showed with all of them to some degree. Primus sacked. Voss sacked. Bucks struggling. Hird, well....
Personally I think it's a combination of Ken's age, life experience and his long apprenticeship as an assistant that makes him successful, on top of being a ripper bloke by all accounts.
Other clubs are seeing the results and are trying to emulate them.