What holds when nothing is chosen. A decision made in advance, on behalf of whoever does not decide.
Old French defaute (a failing, lack), from defaillir (to fail, be lacking), from Latin de- (away) + fallere (to deceive, fail); what holds when nothing else was chosen, a decision made in advance on behalf of whoever fails to choose.
The Old French root is a failing, not a fallback — a default was first the gap left by a missing choice, and only later the answer quietly standing in that gap. What holds by default was decided by someone else, in advance, for exactly the moment a choice never arrives.