While singing Welsh brother Richard and Adam Johnson performed "To Dream an Impossible Dream," a viola player from the backing orchestra rushed out of her chair and began throwing eggs at the judge's table.
The musician, later identified a 30-year-old Natalie Holt, called her behavior "silly" in an apology.
"I want to apologise to Richard and Adam for overshadowing their performance," she said. "I’ve never done anything like this before and in hindsight I have realised it was a silly thing to do."
Holt later explained to the Telegraph that she wanted to make a statement about talent shows.
“I basically took a stand against people miming on television and against Simon and his dreadful influence on the music industry," she said.
"I really don't like eggs," Cowell tweeted after the incident.
He also prompted a discussion about banning eggs on talent shows.
I don't think eggs should be allowed on talent shows. Discuss?— Simon Cowell (@SimonCowell) June 9, 2013