Tuchel basically was brought in to be just a coach - he doesn't actually want to be involved in transfers, so when he joined Chelsea he was extremely happy to work with the hierarchy of Granovskaia and Cech.
When the new owners came in, they completely flipped this - all of a sudden he was being relied upon to not only be involved with but to help lead recruitment, work closely with new owners who kept bothering him with incessant questions, and have tonnes of meetings with agents. He absolutely hated this and expressed his displeasure with ownership, who decided that they couldn't work with him (reportedly one of the flashpoints was Tuchel instantly dismissing the possibility of signing Ronaldo from you lot, since the owners didn't understand why this would be such a terrible idea).