During Galante's 12 year prison stretch Joe Bonnano (a close ally) was disposed by the Commission and Rastelli was installed. This left Galante out of the loop as far as control of the family goes. A lot of guys outside the family wanted Galante dead, but I think the final straw was him trying to take over the family while Rastelli was in prison. Rastelli had many loyalists in the family and Galante had many enemies outside of it. Also it was Bonnano guys that killed Galante (and left his body guards untouched).