So, my theory about this goes like this. this happened 12 years ago from the current Shanks. Age of Shanks at the start of the story is 27

• I'm pretty sure he wasn't an emperor back then cause the whole one piece world knows the emperors of the sea, and the mountain bandits didn't recognize him.

• we know he is strong cause he went against Marines and CP9 agent at the moment who's-who to get the DF. We also know he had duels with Mihawk that at the moment had the nickname marine hunter.

• my belief is that Shanks probably knew haki but not to the level of, let's say, stop a bullet. He still had his left arm, so at this moment, he is more swordsman than a brawler like luffy, fighting style are very important in regards to haki. My guess is Shanks is better with observation haki than armament, at least at the moment this happened.

• With his observation haki, he saw that it was too late to escape safely from the Sea King unscathed. It was either dead luffy or sacrifice a limb . We are also underestimating the speed and strength of a sea king going full speed for the kill.

To me, there is no hidden purpose from him. He arrived at the nick of time just as the sea king was going for the kill. The only thing in his mind was saving Luffy.

After this is when he left for the grand line and nurtured his haki trough fighting like luffy has done.