I think it was Loki who actually started the fight with Shanks, rather then the other way around. If Shanks was motivated to avenge Harald or something, why didnt he fight Loki right after it happened? Or how it took eight years for him to do so?
No, I feel Loki somehow found out Shanks was a God's Blade and felt he had a hand to play in his death and went to fight him based on those feelings. It also explains why Shanks didn't just kill Loki after he beat him.