My guess is it wasn't a design flaw but rather an installation flaw of some sort. It is too soon to tell if it was at the factory or after the plane was delivered to Alaskan Air.
Installation flaw: Someone didn't bolt the hatch in properly when building the aircraft (which was rolled out two months ago).
Design flaw: The hatch, since it's not a door, should be designed so that it can never fit out the opening and the more the pressure builds, the tighter it gets.
In this case, it sounds like it was both a design flaw and an installation flaw.