The humans who edit, close, and vote on questions aren't perfect: They're humans! For this reason, all edits and closures cause questions to be bumped to the top of the 'active' list. This bumping allows actions to be audited. Closing a question isn't permanent. The same number of votes can reopen a closed question. If no one saw these questions, they would be much less likely to be reversed.
After a few downvotes accumulate, a question will be removed from the front page. We don't push it off immediately in case the first downvoters made a mistake. After several downvotes, though, it's almost certainly a poor question and we don't want it on the fron page.
Also note that closed questions with downvotes (and no upvoted answers, if I remember correctly) are deleted automatically after a period of time, so they won't pollute the archive or Google results.