Confirmed: Security breach is not reason for Gatekeeper app signing changes | 9to5Mac
We’ve now confirmed with sources close to the situation that there is no truth to the rumors and that a Dev Center breach was not the reason behind the Gatekeeper app signing changes.

In other words, it's just a worthless change (read: everybody churn!) aimed at making life difficult for 3rd party ISV's which, unlike Apple, do not have unlimited capital with which to hire engineers, do testing and so forth. Should be very popular with enterprise developers.