Where a bounty is not awarded by its creator it is awarded automatically. In this case only half the amount is awarded.
This is covered withing the bounty FAQ:
What is automatic awarding?
Approximately 24 hours after the end of the bounty period, if the
bounty starter has not manually awarded the bounty, the bounty may be
If the bounty starter accepted an answer during the bounty period,
that answer is awarded the bounty. Answers accepted before the bounty
period are not eligible to be awarded the bounty automatically.
Otherwise, if there are eligible answers, the highest scoring is
awarded half the bounty amount. The criteria for an answer to be
- The answer must have been given after the bounty was started
- The answer must have a score of at least +2
- The answer must not have been written by the bounty starter
If two or more eligible answers have the same score (if their scores
are tied), the oldest answer is awarded the bounty.
If neither of these conditions apply, the bounty is not awarded to any
answer, and is not refunded to the bounty starter.