It reportedly all stemmed from the fact Rob is living separately from Blac Chyna until their daughter is born because they don't want their fighting to affect their unborn baby.
The pair are said to have not spoken for months, so Rob's sisters threw them separate baby showers, but Rob then decided not to go to his because he found out it was being filmed.
The sock designer was then told his fiancee couldn't go if he didn't, resulting in him lashing out on Twitter.
A source told Us Weekly magazine :"Here's what happened: Kim threw the shower. Kylie [was] the one that Rob called and told he wouldn't go to the shower, because she mentioned it was being filmed.
"He [then] got really upset and told her he wasn't going, but that Chyna still wanted to go. Chyna was all dressed up and ready to go, [but] Kylie then told him that Chyna wasn't invited if he wasn't going to be there.
"She said it was his shower and Chyna could come if they were together - but if he was going to screw them by not going, then Chyna couldn't come. That's when Rob exploded. It caused a huge fight between Rob and Chyna too, so that's when Rob went off on Kylie on Twitter."