Bounties are awarded by users, and not by some democratic process or measure of correctness.
Bounties may be awarded for any reason, or no reason. The user may decide to award a bounty to an answer simply because it looks like the answerer spent a lot of time on it. They may decide to award a bounty an answer they feel has some particular insight despite not being entirely correct. They may decide to award a bounty to an unpopular answer as a consolation prize.
So you would have to ask the user their personal reasons for choosing to give the bounty to that particular answer.
However, there is the exception when a bounty times out. Bounties have a time limit, and if the user does not specify an answer to award it to, then automatic awarding takes place:
What happens if I feel my question is still unanswered? / What is automatic awarding?
Approximately 24 hours after the end of the bounty period (once the grace period ends), if the bounty starter has not manually awarded the bounty, the bounty may be awarded automatically.
If the author offered the bounty, and accepted an answer that was posted after the bounty was started, that answer is awarded the full bounty.
Otherwise, all or half of the bounty is awarded to the highest-scored answer out of those which...
- ...were posted after the bounty was started, and
- ...have a score of at least 2 (at the time the automatic awarding takes place), and
- ...were not written by the bounty starter.
If the chosen answer was accepted by the question owner, it will receive the full bounty. Otherwise, it will receive half the bounty. If two eligible answers have the same score, the older answer is chosen.
If no answer meets any of the above two criteria, the bounty is not awarded to any answer, and is not refunded to the bounty starter.
For more info on how bounties work, check out the StackExchange Meta question "How does the bounty system work?".