The drama began Saturday at the annual Clive Davis party. According to reports, Fashion Police host Kelly Osbourne, 29, began arguing with Feldstein, 35, after he commented on her brother Jack and his wife Lisa.
When Kelly stormed away, mom Sharon, 61, marched up to the table where Feldstein was sitting with Bareilles. She yelled at Feldstein, threw food onto his lap and then (as seen in this photo) tossed a glass of water at him.
"She was on a screaming tirade," a source tells PEOPLE. "She was like, 'You don't mess with the Osbournes!' " Even after Feldstein walked away, Osbourne was still screaming and security was called.
Adds the source: "It was a big commotion and very uncomfortable to watch."
The next day, Feldstein did not attend the Grammys, where both Bareilles and his other client, Robin Thicke, performed.
Sources say Bareilles dropped Feldstein on Monday, and The Hollywood Reporter broke the story Tuesday.
This isn't the first time Feldstein has made headlines. In November, the Maroon 5 manager wed Clint Eastwood's daughter Francesca, 20, before annulling their union a week later.
At the time, a source close to the Eastwood family told PEOPLE the marriage "was a goof off."
Whatever his intentions this time around, it doesn't look like the Osbournes – or Bareilles – are amused.
Calls to all involved were not immediately returned.