The fire is ultimately started by Mrs. Danvers, the deceased Mrs. de Winter's housekeeper, who is deeply devoted to Rebecca's memory and resentful of the new Mrs. de Winter. She sets the fire with the intention of destroying the house that represents Rebecca, and she also wants to take revenge on the new Mrs. de Winter for taking her place.
The fire is also fueled by the new Mrs. de Winter's own feelings of inadequacy and insecurity, which are exacerbated by the constant reminders of Rebecca's presence in the house. The fire, although started by Mrs. Danvers, can be seen as a symbolic release of the new Mrs. de Winter's pent-up emotions.
The fire is also partially accidental, as Mrs. Danvers's actions are driven by her own emotional turmoil and she doesn't fully anticipate the extent of the destruction. The fire spreads quickly due to the old age and dryness of the house, which makes it highly flammable.
The fire is a powerful symbol in the novel, representing the destruction of the past and the new beginning that the new Mrs. de Winter has the opportunity to embrace. It's a cathartic event, both for her and for the reader, marking the end of Rebecca's haunting presence and allowing the new Mrs. de Winter to finally move on with her life.