1) The coronavirus pandemic is shutting down huge sources of revenue for Disney and can potentially increase exponentially. This is one of the most dangerous times for the company, certainly the most dangerous since 9/11 and could end up being worse.
2) Lucasfilm is moving into dangerous territory for profitability and Chapek does not have the acumen or the gravitas to remove Kathleen Kennedy... Iger did.
3) Fox movies inherited into the Disney company have been almost universally losses and look to continue damaging the company's bottom line. And given that these properties are generating losses, that indicates systemic issues in those studios.
4) Chapek is simply not viewed as anywhere near competent to run a company of this scale. Chapek couldn't run the Parks division and has a shoddy track record. With this sudden announcement out of nowhere in the most volatile time period the company has faced in decades, the stock price could take a huge hit tomorrow morning.
5) There is fear Iger wanted to "lock in" now, suddenly and abruptly.
6) Marvel's future is not as strong as it was leading up to Endgame. The future slate of movies is not viewed within the company and the industry as being strong, and there is significant push to get X-Men up and running... but so far that's not moving as quickly as many would like. Iger was able to form a great partnership with Feige in making Marvel a huge success - Chapek does not have that relationship, and will likely get run over.