Obelisk targets minions first, summoned by ryze

I was playing a game on TT, and then my teamate playing ryze used his ult at the enemy nexus turret to the spawn point of the enemy. we killed the enemy who had just spawned, but the obelisk was not targeting us at all. we did deal damage to them, but the obelisk didn't target us until the super minions that were brought with us were dead.