Minion Aggro... whut?

Your minions are hitting the enemy minions, so you know when an enemy minion will have enough HP for you to last hit. Then suddenly your minions switch aggro for no apparent reason, no champion hitting them (I know for sure because I was testing in a custom game), no turret nearby, there is nothing to disturb the simple process of last hitting. So what gives? I look up at the Wiki about minions and there was a priority list, and the most important there was the 3rd dot: Minions hitting minions. But taking all of this into consideration things are still unclear.

Minion aggro is incredibly obscure despite being such a common game mechanic that can really affect your farming. Here are a few things that I do not know:

• When minions get hit they send out a "cry for help", but there is also a part that mentions that minion scan every few seconds for threats. This is unclear, will a minion act upon a call for help only if they "hear" the cry for help during a scan? How often does a minion scan? Does a minion send out a cry for help every time it is damaged? If the first question is false then will a cry for help be considered a higher priority than the threat scan?
• If only 2 melee minions are fighting 2 melee minions and they are duking it out 1v1, as soon as a minion gets hit it should send a cry for help and the allied minion should attack the other aggressor. This would result in a permanent target switching, do minions respond to their own calls for help, meaning they will not respond to an allied minion's call for help as long as they have their own enemy attacking them?

I will continue testing but I wanted some answers to these question because during some custom games things didn't quite happen as I was expecting. I know this is almost impossible due to every minions being a variable making it too complex but the main farm stealer in the game aside from the support and the turret is the siege minion so figuring those guys out should be enough. Ranged minions are also an easier thing to figure out since they have less interaction due to being ranged.