She has definitely had the whole thing planned out, presumably in exactly the same way she plans her books!
She just needed someone, anyone, to be a thoroughly obnoxious oblivious to how much hate they themselves were spewing out on twitter in JKRs direction person. And if that someone was also desperate for JKRs attention and was itching to report the nasty lady for calling them hurty words all the better for the plan to work.
I'm sure she had loads to choose from, but only a few will have tweeted the right things at the right moments in time and could be guaranteed to react in a totally OTT self absorbed way.
And we all know who the lucky person was!
It was kind of inevitable it was going to be IW though as IW is very predictable in IWs behaviour patterns!