I don’t have an answer for you, but I know that I have previously read complaints on FT about people getting bumped from seat 1A on different airlines and a variety of aircraft. Usually this pertains to international flights (unlike your domestic flights) and the reason/best guess offered is that the seat is used for pilot rest. On a domestic flight, is it possibly a bulkhead seat for which priority might be given to someone with a disability?

Again, I can’t explain the reasons for this and I’m sure there are many people here who have flown in seat 1A dozens of times without getting bumped. I just know that I’ve seen enough of these complaints that I do not choose 1A on any airline if there are other options and I am particular about selecting my precise seat. My general sense is that if airline ever has a reason to reserve a seat for some kind of special passenger, 1A is the default option.