FWIW : sometimes when I type or paste lines of plain text I get each line wrapped in <div>... tags, other times they're in <p>... tags, and other times they're in <p>... tags within a single <div> tag.
And on yet other occasions when I paste plain text, it gets wrapped in apple spans, verdana font tags etc, etc. And I know for certain the clipboard contained no HTML markup because I have to go out of my way to disable clipboard markup scrubbing.
I've never been able to fathom a repeatable pattern, hence I've never reported it, but I have a suspicion it may be conditioned by previous edits of long text cells within the same session.
Added : The only occasions I can recall seeing <p class="description"> is after using Terisa's Clean Comments PI.
BR