Why are the minions set for hero focus???

I watched an intro video for this game where a guy talked about minion aggro. He said minions only choose you as a target if you are the nearest enemy or you attack a nearby enemy champion. Well, the following happened today:

I played with a melee champion (Katarina) against a ranged champion (Karthus) so I stayed back and didn't do anything until the first minions weren't low enough to use my Q and finish them. That was the first time I stepped up. However, when I got in Q range to 2 enemy minions, both of them stopped attacking the minions they were fighting until that point and targeted me instead. Both group of minions were equal in numbers and I wasn't even close to the enemy champion. The moment i was close enough they focused me for no reason.

I experienced similar things earlier, but this one was an unquestionable proof. So 1, why did Riot changed the settings or 2, why is that guy lying in the video?