What are Near Instant Blacklists?

Why did all 64 of my IPs end up on Spamhaus after my first blast, even though I was sending to highly engaged opt-ins, had only 11 hard bounces out of 35,000 emails sent, and was rotating subject lines, content, IPs, and domains at a rate of 2 emails per minute per IP? What am I doing wrong that is causing me to get blacklisted so quickly?