This just happened here. What are the precise rules with which the auto-awarding algorithm operates?
From the bounty page in the help centre:
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.
The answer to your question was posted after you started the bounty and has a score of 5, therefore qualifying for the algorithm giving it 50 reputation from your 100 rep bounty.