Here are all the reasons a post could end up on the top of the "Active Questions" list:
- New question (also visible at top of "Newest Questions" list), which hasn't gotten any attention/answers at all yet.
- New answer (to an old or new question), which users should vote up or down to show whether it's a good answer or not.
- Major edit to clarify or add new information to a question or answer, that might make users want to revise their original up/down/close/delete votes.
- A question that was closed (and therefore, didn't get any answers before) has been fixed, and so it's reopened.
- User added bounty to question to get it more attention (also visible on "featured" questions list)
- Unanswered old question is automatically bumped by Community user to get it more attention.
- An edit to an old question that already got a lot of attention and has good answers (or an edit to one of its answers) that is a non-trivial edit or retag that adds value. This is bumped to the top so users can review the edit and make sure it was correct, and revert it if it was not.
All of these kinds of posts/edits need some kind of attention from other users, and should be bumped to the front page.
But the things at the top of the list are generally considered more in need of attention than category #7. It's fine (even good!) if a few questions in category #7 are bumped here and there, but not if it's so many that most of the questions in categories #1-6 are pushed down the front page.
For example, imagine you spent 45 minutes composing a great, well-researched, targeted answer to a question. Then, immediately after you submit the answer, someone else bumps 20 old questions to add a tag. Nobody sees your answer now that it's all the way down the "Active Questions" list, and you don't get any upvotes for this amazing answer that you worked really hard on. This is obviously discouraging and demoralizing, and makes a good contributor not want to contribute anymore.
Or imagine you're a new user, and you posted a question because you really want an answer. But the question needed improvement, and was downvoted and closed. You work really hard to understand why the question was closed, and put in a lot of effort to improve it. Finally, after four days of working on the question, it's reopened! But 20 old questions that were edited right after your question was reopened have bumped yours all the way down the "Active Questions" list so it doesn't get any answers. As a new user, you feel discouraged, and wonder if the time and effort you put into learning how to use the site was a waste.
On the other hand, if those 20 old questions were spaced out a little so they didn't flood the front page and knock posts in categories #1-6 too far down, I can't think of any negative effects.