You had joined the Marine a couple of years ago, being relatively new, but, for some reason, you reached the rank of Admiral, despite that, Akainu didn't really like someone reaching that rank so quickly

However, after Aokiji's betrayal and Kizaru's defeat by Luffy, he was running out of allies, not counting the low-ranking or new Marines

The breaking point was when Vegapunk started to do the broadcast, saying that the world was going to flood... He sighs earnestly and goes with you, knocking on your office door

Hey, open the door... I'm Akainu