Fresh from the Mouth of AT&T, who like his Lord of the Rings counterpart, wears creepy masks at inappropriate times is AT&T's official reason they blocked 4chan. It was, as suspected, blamed on a DDoS attack from that IP address:
Beginning Friday, an AT&T customer was impacted by a denial-of-service attack stemming from IP addresses connected to img.4chan.org. To prevent this attack from disrupting service for the impacted AT&T customer, and to prevent the attack from spreading to impact to our other customers, AT&T temporarily blocked access to the IP addresses in question for our customers. This action was in no way related to the content at img.4chan.org; our focus was on protecting our customers from malicious traffic.
Overnight Sunday, after we determined the denial-of-service threat no longer existed, AT&T removed the block on the IP addresses in question. We will continue to monitor for denial-of-service activity and any malicious traffic to protect our customers.
I'm sure that won't stop the 4chan retaliation. [AT&T]