I honestly think she got fed up with Ashy leaving Taj with her every weekend and whenever they went on holidays. So rather than talking about it, they just decided to move up north. That might seem extreme to some people, but I know for others they would do anything to avoid a confrontation. Perhaps they had wanted to move for awhile and Ashy's constant demands just moved things along a lot quicker.
I remember Ashy doing a big rant about how she felt she was giving her mum a gift of allowing her to look after Taj. Look, I'm sure she loves him, but she's not his friggen mother and she probably got over every weekend being expected to have him and for their numerous holidays.. and remember back then she was going for a few weeks at at time.