Minion arbitrary target swap

Everyone has seen it. The minions will be happily attacking one of their different colored foes, then suddenly swap to a different target. Sometimes a melee minion will stop his heated duel with another melee minion, because he decides he wants to bash a caster minion instead.

To be clear, I am not talking about when the minions are protecting a champion that is being agro'd. Just when they swap on their own for no observable reason.

Can anyone shed some light on this?