The rise in quality occurred under Shooter. The decline began almost immediately after he was fired. That's pretty conclusive, right there.

Yeah, a few guys like Byrne and Miller butted heads with him. But if you look at their work for Shooter and their work after, most of their best stuff occurred while working for Shooter (the only exceptions being Miller's stuff on Batman in the mid-80s).

Shooter seems to have been lacking in people skills (probably because he came up under the legendarily abrasive Mort Weisinger at DC) but unlike the EIC's who followed him, he understood story structure and insisted on it.