I find it hard to believe it was accidental. A button like that would be THE thing that every employee is told about to NEVER push unless nescessary.
I think it's more likely a case of either disgruntled employee, or some idiot that thought it'd be funny, or made a bet with friends.
Eh... official-ish line I've seen is that someone (maybe newish?) autofired their way through a yes/no prompt. That thing a lot of folks that catch the occasional virus or malware do where they click buttons on autopilot during an installation, basically. Seems like they've already changed it to need two-stage authorization, for what that's worth, or whatever the hell the word is where it needs more than one person. Gods know if the rapid implementation is going to cause bugs in the near future.
It apparently
was part of a fairly common drill, but so far as pretty much friggin' anyone (outside
maybe the joint that triggered the alert) was concerned, no one knew it was for a good chunk of an hour. Y'know, 'nuff time for a bit of the state to get glassed.
... that said, this incident did just tell the world it would probably be real fucking easy right now to launch a decapitation strike on the US. POTUS, on yet another tax funded golfing trip, was apparently either not informed or didn't give a fuck one of our states (and one of the ones most likely to actually eat a hit, at that) triggered an incoming probably-nuke warning. While hawaii was re-experiencing the cold war, 45 was golfing.
There's the morbid side where folks are relieved he wasn't involved and there to do something unfortunate, but it at least says we have goddamn serious procedural problems in the top levels of the gov't at the mo'.