Our relationship was a very unhappy one (marked by significant emotional abuse on her side, 'borrowing' of large sums of unreturned money, invasion of my privacy etc.), which 'culminated' in my slapping her and her successfully prosecuting for me for common assault - a unique event in my life.
(She is, as I too late discovered, a pathological liar with an undiagnosed personality disorder.)
Her motive for the false allegation is difficult to determine, but most likely an expression of her general hostility and nastiness three years on, exacerbated by her seeing me with a new partner.
My partner and I protested very vigorously to museum staff, who simply refused to act on our (correct) version of events, citing a spurious 'duty of care' toward her, though we were at pains to emphasise we had no malicious intent and would not even have been in contact with her during our visit. (They also called police, despite no criminal activity having taken place. The police duly confirmed there was no restraining order, yet we did not even receive an apology.) In a public space, and at a museum I love and have visited many times, I consider the damage to my reputation and hurt feelings to have been considerable.