Sources close to Jon and Kate tell TMZ that Kate was upset because Jon wanted to leave the kids with Stephanie Santoro -- the babysitter who was handpicked by Jon.
We're told Kate has serious trust issues with Santoro, and doesn't want her playing nanny with her kids. Kate went to the house to watch the kids instead of Santoro -- and that sparked the fight that ended with Kate calling 911.
Jon pretty much confirmed all this -- in his words -- this morning at his house. When in doubt, blame the nanny.