Not because of any devious dark magic; apparently, the cause is that boring old spell known as “creative differences.”
The director himself took to Twitter to confirm his departure from the project: “Marvel and I have mutually agreed to part ways on Doctor Strange: In the Multiverse of Madness due to creative differences. I am thankful for our collaboration and will remain on as EP.”
Variety reported that Marvel released a rather similar statement of its own: “Marvel Studios and Scott Derrickson have amicably parted ways on Doctor Strange in the Multiverse of Madness due to creative differences. We remain grateful to Scott for his contributions to the MCU.”
Other than the 2016 first Doctor Strange movie—starring Benedict Cumberbatch as the talented surgeon who transforms into Marvel’s Sorcerer Supreme after a hand-mangling car accident sends him down a more mystical career path—Derrickson’s directing credits lean heavy into horror, with such films as 2005's The Exorcism of Emily Rose and 2012's Sinister.
He is also listed as a producer on the Snowpiercer TV show, though he ostensibly parted ways with the series after a showrunner shake-up in 2018—that’s a whole other saga you can read about here. Snowpiercer is set to premiere on TNT sometime this year.
Doctor Strange: In the Multiverse of Madness is slated for a May 7, 2021 release in the U.S., and the Variety story notes that Derrickson’s departure is not expected to delay the start of shooting in May of this year. There is, of course, now a pressing need to hire a replacement—which means the speculation on just who might step in may now begin.