In the early 90’s in Atlanta once they had someone putting up K’s for each strike out. When they got to 3 strike outs it said KKK. Someone thought this was an advertisement for the KKK. SO ever since then is KK and 3rd K is a reverse K.
Okay, I am nervous again...
Ah, no: it comes from how you fill in your scorebook. A regular “K” is when the batter swung at strike 3, a backwards “K” is when the batter watched strike 3 without swinging.