The simplified timeline is this.
Summer of 2018.
Everything is great. Messi is happy, recently extended, and promises the CL title this season. The veterans love Valverde's handling of the team and everything is peachy.
Summer of 2019.
Another historic humiliation vs Liverpool destroys what was looking as potentially a great season. From hero to zero so to speak. Team and Valverde under fire. Messi takes the blame on him and the players. Supports Valverde full on.
Also, in that summer, numerous news regarding the pursuit of Neymar, and bringing him back. Also some news suggesting this was asked by Messi gained traction in the media, even from reputable sources (probably they talked with Neymar and they saw he was open for a move back since he was having a really bad time at PSG at that point). More leaked news that the board trying to get Neymar was just an act to please Messi, when in reality the operation was not feasible because of PSG not willing to let him go, and Barca not being able to afford such an expensive operation after splashing all the money on Coutinho and Dembele.
Winter 2020
Team was going from worse to worse (basically a continuation of the downward spiral that started on Anfield), and Valverde was sacked. Messi starts to attack the board (Abidal) for disclosing that the main reason for Valverde's sacking was that some players accused the poor intensity in trainings (something which was reported multiple times from more than one source). He wants names (lol). Messi also starts to rebel against the newly appointed manager and his staff. News saying the heavyweights of the locker-room didn't like the changes Setien and especially Sarabia brought. Live video vs Celta shows Messi completely ignoring the coaching staff and walking away from any instructions.
Summer 2020
Historic humiliation vs Bayern, followed by major changes. Messi's best friends in the team on the chopping block.
So, we have 4 things that majorly upset Messi in my opinion.
1) The sacking of Valverde even though the veterans loved him and publicly supported him avidly;
2) The fake pursuit of Neymar, a player Messi asked for to improve the team;
3) The arrival of Setien and Sarabia, who, without having good resumes tried to rock the boat and change things;
4) And, the final straw, the club turning against the veterans, especially Suarez. And hiring a no nonsense manager who is willing, seemingly at least, to re-impose a model based on hard work and discipline.