I have seen it few times in big corporations I worked in. Somebody sends email to wrong group by accident and then we have 3 waves of attack:
1) Clueless people hitting 'reply all' asking for removal from mailing group
2) Even more clueless people hitting 'reply all' asking people to not 'reply all'
3) "Champions" trying to save a day by putting all in BCC and telling people to not reply all, unless you put it in BCC 
And then, few hours later, next timezone wakes up and things start again.
Why is it useful? After it is obvious what is happening, you create folder called 'idiots' and redirect all these emails into that group by outlook/whatever rule. After that, if you need to deal with somebody in your organization, first check if he/she is in idiots folder and approach accordingly.
BTW, 120 replies seems very low. I have seen mailstorms with group of 10k recipients (it was not 'all' group, just some subset of company) generate over 600 replies total in these 3 waves. 120 replies from 1.2 million looks to be technical limitation (or, maybe, there was some hero in IT department who pulled the plug fast enough...)
 - My favorite is self correcting champion, which first sends 'reply all' and then does reply to that with everybody in BCC saying he should have put everybody in BCC in first place...