What are the possible reasons for Scrooge’s change over the course of ‘A Christmas Carol’?

I would argue the three main catalysts for change correlate for the 3 Christmas ghosts: the Ghost of Christmas Past makes him remember feelings of love and joy, the ghost of Christmas present makes him rethink his idea of justice, and the Ghost of Christmas Yet to Come (Future) instils fear in Scrooge. In short Scrooge is motivated to embrace the Christmas spirit of love, joy, justice, charity, and to move abruptly away from the future he fears.  

The other ghost, Marley, also fits into this pattern, in my opinion relating most closely to the Ghost of Christmas Yet to Come because both ghosts terrify Scrooge and without this fear pressuring him to listen and learn he would not be capable of learning about love or justice.