From the help center instructions to those who offer a bounty:
If you do not award the bounty within 24 hours of the bounty period ending, half the bounty value will be automatically awarded to the top voted answer posted after the bounty start, provided it has a score of at least 2. If no new answer matches this requirement, no reputation will be awarded at all, and the reputation used on the bounty will be lost forever.
If the full bounty amount is awarded -> the person who set the bounty awarded it.
If half the bounty amount is awarded -> the software awarded it. The person who offered it did not award it to an answer within the deadline, despite several reminders.
Also see How does the bounty system work.