Of course he acted on his own volition (unless Blinkee had photos on him for blackmail). I was not disputing that, but saying that Blinkee asked him to write the deceptive letter.
Most likely, he said to Morrell something like, "If only there was something like a letter signed by a large group of intelligence officials that the campaign could use as a cover to keep this out of the news and give cover in the debate." As scheming, dishonest hacks, they would know to speak like that so they could deny it in testimony.