The official reason cited by both parties (Lucasfilm and Lord & Miller) was "creative differences" – everyone's favorite vague excuse after "scheduling conflicts."*
But parting ways in development or pre-production is one thing. That happens all the time. It's practically unheard of for such a dramatic exit to occur five months into shooting, with only weeks to go. As my colleague Chris Taylor put it, "whatever 'creative differences' Lord and Miller had with Lucasfilm must have been the size of Starkiller base." Read more...