To go mental is to be mad with rage, to be out of control with anger, to be ranting with wrath.
In other words, to be making a complete idiot of yourself.
(Anyway, why follow the crowd?)
Someone or other, whose name I hardly dare not type for fear of the baying mob (but it was Kipling) begins his poem If:
If you can keep your head when all about you
Are losing theirs and blaming it on you
(later on in the poem there are the lines If you can bear to hear the truth you've spoken/Twisted by knaves to make a trap for fools, which is sadly relevant to the times, if not this post).
The poem ends you'll be a man, my son...
...but, oh dear, that word man...
...that's going to whip up a lot of hysterical fury among the differently-sexed, isn't it?
People will be going mental.
Thing Not To Do Today: go mental. This word comes from the Latin word mēns, which means mind.