And now we know why!
The new explosive teaser trailer begins with the three sisters fighting, as Khloe asks Kourtney, "Why do you have to have an attitude?"
"Don't involve yourself in business that is not yours," Kourtney replies. "Then don't talk about it in front of me," Khloe shoots back.
Later, Kim calls Khloe to tell her something HUGE about her ex Tristan Thompson. "You're gonna kill me: Tristan is joining us for dinner," she tells Khloe.
But what we've all waited for is toward the end, when things get physical between Kim and Kourtney.
"You have nothing to say!" Kim yells before grabbing her oldest sister. "Don't ever come at me like that!" she says as she raises a fist to Kourtney.
"Buttons have been pushed," Kendall Jenner states.
Season 18 of Keeping Up With The Kardashians debuts March 26