I would have Placerias start breeding in an enclosure not too far away, with eggs and babies sprouting about - only to visit my trusted Ischigualastia mount, who unexpected attacked me. A Camarasaurus that I had wander about was also unusually aggressive as well, plus my Ankylosaurus who is usually passive due to me imprinting on all of these animals.

I had to kill off the Placerias for a moment and take their eggs, which caused all the animals to finally no longer be aggressive.