Your guess is as good as mine for his departure.
Just announced this week, Trump's former pick to lead FEMA resigns from agency
Why is he leaving? Who knows? All we know are the facts that he was announced as the new FEMA administrator and the background investigation for his appointment turned up information that killed his chance to serve.
One thing I'll point out is that you need to be careful who you associate yourself with. In many circles of people, there are those who judge the entire Trump administration to be crooked and corrupt. That is, of course, not true in my book. Take the current FEMA administrator, Peter Gaynor. I knew Pete from when he was "just" the Emergency Management director of Providence, R.I. As I shared with someone else this week, he is a "straight arrow" type of guy. Far from the well-coiffed long-serving Washington, D.C., insider who you can spot from a mile away when they are out of their element — here in the field.
I expect that the departing Jeff Byard will end up at some consulting company. Isn't that where everyone seems to end up?