How should I frame my transition from an unsafe situation as I apply to other universities, pre-tenure? How do I frame my exit from a state university under duress in applying for new jobs ?
I am pre-tenure in the United States, and I find myself needing to change jobs because conditions at my University re: COVID have gotten to the point where they threaten my safety, and that of my family. I will not go into detail about this. The situation become so negative that my chair has approached me to offer their support for either my staying, or support through writing me letters of recommendation to leave.
I am in a strong position academically, and tenure is all but guaranteed in two years (including one year added for covid).  I am currently writing for multiple positions.
How do I frame my reasons for applying for new jobs, which is entirely related to the behavior of my state, less so to my university, and not in the least to my department, which has worked hard to protect and help?
As a small additional question I wonder if I even want the letter from my chair, as it may invite questions?
Your personal circumstances should not affect your attractiveness as a candidate.
People move for personal reasons all the time: spouses getting better jobs elsewhere, proximity to better medical care or schools for their kids, less taxes, affordable housing, or because the weather sucks where they are now (shoutout to the British Isles!).
This question may arise in personal conversations when you interview, but should not be a factor (or even mentioned) in your application materials.
If your department head is supportive and your move is motivated purely by personal reasons, then you should have them write a letter.
I would advise against getting into details that may present your current institution in a negative light. Interviewers like candidates who present themselves as team players. Statements like “the university put me in a situation that threatened my health” may come across badly. Try to word it a bit more gently: “I heard that prospective institute name has excellent healthcare and this is a factor in my decision”.
Good luck!