I've noticed this on some comments I've made, I'll sometimes get 8-10 upvotes in the first minute after posting the comment.
The only thing I could figure was I was using keywords some how. This happened once when I was having a conversation with someone and I got nearly $3 in that debate, more than I get on my posts.
I've also had a couple posts that had a "school" swim by and those posts made $100-$200. At least with those one person may have read the article originally because the votes came a while after posting.
I admit appreciating the rewards, but it's so random you don't know how to actually build something, and you question if you can really develop an upvoting readership.