No, it's not a bug. Basically the system looks for questions that have at least one answer but none that are accepted or upvoted. It then performs a 'non-edit' to bump them to the top of the active view with the hope they attract attention and get 'resolved'. It's basically the system trying to clear the Unanswered view - an upvote on any answer removes it from that category, even if not accepted.
Here's a SE Meta post that has the details: What can cause a question to be bumped?
• The Community user will bump non-negatively scored questions that
have at least one answer scoring 0 and none scoring more than that.
• Questions are picked randomly from the top-viewed inactive questions.
• The number of unanswered questions bumped per hour depends on the
site: 4 per hour on Stack Overflow, 1 per hour on Meta, and 2 per hour
on Super User and Server Fault. The default for a new site is 1 per
More info about the Community user can be found at Who is the Community user?