Well, he did lose the locker room. Right before he lost the locker room two things happened:
1. We were playing bad and risked to miss out on direct CL qualification, with a small chance to even lose the CL qualification.
2. He benched Del Piero for 2-3 matches.
Any of the two (or maybe even both together) could be the reason why he lost the locker room and losing the locker room might have been the main reason why he was fired (although, tbh, it might have not been the reason. Though that's a smaller chance).
We don't know that yet. Ranieri indeed never went public even if in disappointment he said that he will reveal the truth.
I did come to the same conclusion as you did, though.
But no proof yet