I believe it was his decision to dump Sam Darnold. Now mind you, Darnold wasn't that great as the Jets QB, but he should never have been dumped, even if the Jets were planning to still draft Zach. Zach should have been given time to slowly transition into the full time starter instead of being the Day 1 starter.
Instead, Darold is traded, he has gone on as a career backup now but is looking decent enough that he just beat the Jets. In fact both times he has been the starting QB vs. the Jets, he has beat the Jets. He was the starter for the Panthers, again a terribly ran organization and his stats were bad. The fact is, Darnold isn't a bad QB... he got screwed, like Zach did, by the Jets.
Another scenario is that the Jets decided to keep Darnold as their starter and build around him, that would have given Zach a better chance to land some place else, like maybe San Francisco or something.
But Saleh's entire decision making around Darnold seems to have set off a chain reaction of just bad decisions that have lead to hurting the careers of 2 1st round draft pick QB's and even a veteran QB in Rogers couldn't save this crap decision making.