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The relationship between Trump and Bannon seems to have been a complex one,. He was savaged by Scaramucci (who wasn't) and had been pushed to one side, then brought back, etc
He shoots from the hip, and maybe there's only room for a limited number of those in Trumpworld. Of course he did, in a very recent interview, brand the far-right in the US as a "collection of clowns" and "losers". Despite actually being, in many people's eyes, a representative of that group. Maybe that was the last straw (dives for cover).