Probably he was told he wouldn’t be re-appointed. Which is normal for an incoming administrations, who normally wants to “reward” their supporters with “plum” appointed positions. He goes and complains to Zero. So Zero ‘fires him’ since only Zero at this point had the authority to do so. The compliant press then re-casts this as an “incident” where Trump exceeding his authority. They hope to spin it into something “racial” since this guy is black.
Another possibility....
General Schwartz might have been pro-Trump. He might have let it slip that he was glad to see Trump coming to the White House or how happy he was to serve on Inauguration Day.
Word got back to the Petulant Child. Who then fired the good General.
Now THAT I could believe. Obastard is that petty.