The emergency notice was triggered after an “employee pushed the wrong button” during a shift change at the state’s emergency management agency, Governor David Ige said at a press conference in Honolulu.
I'm aware that he said that, and that may be the actual explanation (let's hope so), but even then there is work to be done - maybe adding authentication or confirmation to it etc. That said, this is a situation where if it were more serious (someone hacked in, for example), they'd almost certainly lie about it.
Source: http://fortune.com/2018/01/13/hawaii-false-missile-alert/