default_reasons seems like an odd choice of name, when there is no other kind of reasons. I'd suggest something like reason_choices or reason_options.
Edit: also I'd make it a tuple rather than either a dict or a list.
Edit: also I'd make it a tuple rather than either a dict or a list.
If nothing else, use lists because the syntax means it stays as a list even if you someday remove all but one option and forget to leave a comma, eg.
REASONS = [
'leaves on the line'
]
REASONS = (
'leaves on the line'
)