What is going on

So today I've fought a megalodon that took over half an hour to die. I hit with 15 cannon balls and 60+ pistol/sniper rounds before it finally died (solo)
Later I joined my two brothers and we stumbled across a kraken and took out between 7 - 9 tentacles before it decided it had enough and left. We never killed it after taking out all those tentacles. Why what is going on at rare?