They're not from Dreamwidth. They precede my involvement therein, and I've made maybe 3 posts there. I just have friends who use it so I go look from time to time and it's more convenient to have an account to log in from.

My guess is it's automated. The bot finds a livejournal base address (in my case kayshapero.livejournal.com), then ratchets forward through six character numbers point html in whatever algorithm suits them until a page opens, then uses the comment feature to drop a few posts consisting of lots of links advertising what are either sales of dubious knockoffs (Ugg boots appear to be popular), or trapped links - I've no idea since I NEVER click on them.