He is trading votes. Forming a colluding group is one of the main abuse vectors besides self votes and selling votes. Read the whitepaper for more information about this.

We don't have enough SP to go after each and all abusers at once, so we focus on a few. When they stop their abuse, we move on to the next. Or not, when they don't ;)

Every downvote for you takes away curation rewards from the people you trade with. So in the end it doesn't really matter who we hit, but of course we start with those where we think the chances to make them adopt a more community oriented behaviour are the greatest.