What does Mr.Logan mean when he says, "This thing's been coming a long time, baby, and T.J. just happend to be the one foolish enough to trigger it"?

Mr. Logan is referring to the show down between the white racists and the black tenant farmers. With all the tension between the two factions over land, money, and boycotts, sooner or later the fight had to come to a head. It had to end in one way or another. In the end, Mr. Logan insures that it will end as peacefully as possible.