For this specific case: "Hot Network Question" effect.
Based on the question's timeline, it became a hot network question on Jul 4, 22:46Z when there were 2 answers already.
Hot network questions are shown on almost all Stack Exchange sites, so this question got visibility network-wide. Then, visitors who have a rep association bonus (+100 reps) have enough reps to upvote, so they likely upvoted the question and its existing answers (the top 2 upvoted answers).
Meanwhile, the answer with the source was posted on Jul 6, 13:43Z, almost 2 days after. However, the duration for hot network questions is limited to 2 full days only (i.e. until Jul 6, 22:46Z), so that answer only got 9 hours from increased visibility. After that, it returned to votes from the site regulars and natural visitors again.