Tower targeting mechanics.

I was playing a match and the opposing team dove the tower. Soraka auto attacked me and got the tower to aggro on her. Jhin followed up. I'm not sure how the mechanics work but i thought if a champ attacked you the first champ to do so took aggro. Such as Soraka being focused by tower. However as Jhin was still attacking me Soraka was out of tower range or dead and the tower switched to a minion. Then after Jhin's second attack on me with the tower focusing the minion it finally focused Jhin.

  My question or suggestion is why did the tower switch to a minion first rather then Jhin. Surely it was luck that jhin just happened to be between shots when Soraka got out of range, But jhin was attacking before the tower stopped attacking Soraka. So why was he not a priority target and the minion was. So if riot does not have a If statement within thier coding that would keep track of priority targets before attacking a minion then i would like to suggest one.