Elisabetta Grillo has said that the incident where Charles Saatchi was photographed with his hands around Nigella Lawson's throat prompted her to change her defence.
Asked why she did not mention Ms Lawson's alleged drug taking as part of her defence until last month, Grillo added: "I didn't want to use it before because I wanted to protect her.
"I think especially when Charles picked her nose, it was the proof she still took drugs and he discovered that day.
"So we then decided it was the moment for everybody to know the truth - she could lie easily."
Celebrity cook Nigella Lawson said she was "disappointed but unsurprised" after her two former personal assistants were cleared of fraud.
The revelations that emerged in the fraud trial of two former aides to Nigella Lawson are unlikely to damage the TV chef's meteoric career.
Nigella "let the children smoke weed," an ex-PA has alleged, admitting she had wanted to drop drug claims out of sympathy for her ex-boss.