The answer in question received the bounty because it was posted after the bounty was started and received a score of 2+, having the highest score amongst eligible answers.
If two answers are tied in score at the end of the bounty period, the oldest is awarded.
From How does the bounty system work?, under
What is automatic awarding? (Bolding mine):
Twenty-four hours after the end of the bounty period, if the bounty
starter has not manually awarded the bounty, an eligible answer can be
automatically awarded half the bounty amount. The criteria for an
answer to be eligible for automatic awarding are as follows:
The answer must be given after the bounty was started
The answer must have a score of at least +2
If two or more eligible answers have the same score, the oldest answer
is awarded the bounty.
If no answers meet these criteria, the bounty ends without it being
awarded to any answer, and is not refunded to the bounty starter.
An accepted answer that does not meet the above criteria is still
ignored by the automatic-awarding system unless it is accepted during
the bounty period. An answer accepted during the bounty period will
automatically be awarded the full bounty.