Answering a question marks it as active, but are there any other actions that can do that (i.e. voting, commenting, flagging, reporting, etc)?
See this FAQ post on Meta Stack Overflow: What can cause my question to be bumped?
The following will cause a question to be bumped:
- Addition of an answer
- Edit or retag of the question, or edit of an answer[...]
- Addition of a bounty (by anyone)
- Successful reopening (if the question was closed)
- The Community user will bump unanswered questions every hour. The number of unanswered questions bumped per hour depends on the site [...]
(The re-open bump is a fairly recent development, or rather an old one that was recently fixed.)