On September 6th, 2013, the automatic triggers for putting questions into the reopen queue changed. They were tweaked repeatedly over the years and, as of December 20th, 2019, a closed question will automatically be added to the reopen queue when it is...
- ...Edited (body edits only) within 70 days of closure by the author. Or,
- ...Edited (body edits only) within 70 days of closure by a 3rd-party, provided the editor has not also flagged the question or voted to close it. Or,
- ...Sufficiently popular, where popularity is calculated based on question score, top answer score, or views per month.
Note: for #2 (3rd-party edits), any flag raised by the editor other than "moderator intervention" disqualifies the edit from consideration, even when the flag comes after the edit, is declined, or is retracted by the flagger. Flag types include close and reopen votes, which are represented as flags internally. There are some unintended consequences to how this is handled.
As always, a reopen vote will add a question to the reopen queue if it isn't already in the queue.
Previously, if the author edited a closed post within 5 days of it being closed, that would trigger it to be added to the queue. This was a great way to get additional views for questions that might've been improved enough to be re-opened, but it didn't do much for questions that 3rd-parties without the ability to vote for reopen think should be re-opened.
So the heuristics mentioned above are effectively a way of avoiding yet another flag and thus requiring two actions to enqueue a question: they are things that might indicate a closed question that could use some additional review - or if nothing else, at least a question that someone still cares about.