Harmon was replaced by writers David Guarascio and Moses Port on Community in May, just two months after he had engaged in a public feud with his star Chevy Chase.
Offerman - who plays Ron Swanson in NBC's Parks and Recreation - was asked about Harmon's firing in an interview with GQ.
"It was crazy, but not surprising. Dan has been notoriously difficult with NBC," Offerman revealed.
He added of Harmon's behaviour: "He had that really public Chevy Chase feud. I think Dan is brilliant, but we all kind of hung our heads and thought, 'That's no way for a boss to behave'."
Harmon's departure has not affected the cast of Community, who with Chase have all signed up for the fourth season.
Community and Parks and Recreation both return to NBC in the autumn.
Watch a clip from Community below: