You don't stop building links even if you're #1.
Anyway you might have had alot of links from one site due to page interlinking and the comment page got archived.This is quite common.Now if you blasted at multiple pages on the same site,you can imagine how many links you had coming from it then for that to stop basically will cause what has happened.
ALWAYS only have one page per domain,you can harvest more urls for web 2.0's from these sites but I learnt this a while back the same way.I made one post and had 10k of links from it due to page interlinking on the site.
Hopefully you've been checking the number of backlinks regularly,if so you'll see if this is the case a mile off.