She didn't actually break them up, but she was the catalyst who brought everything to head. They had already been arguing about who was going to be their business manager. Paul wanted Linda's father, but the rest wanted Allen Klein. This upset Paul, feeling as though his mates had turned against him. Also an unspoken agreement that girlfriends and wives were not allowed in the recording studio, which John ignored. She frequently inserted her unwanted opinions into their music, which Paul and also George felt she should never do. And John also made frequent and disparaging remarks about Ringo's talent, Paul's insecurity.and wanted out. It was happening before she came along, but after she came, John was done.