Minions move too fast and don't care about you

So you're at top, sitting by a large group of minions you built up, and you slay the last couple melee minions, that your melee minions were fighting.

For some reason, if there are 'no enemies left' (Rengar is in the bush btw), after slaying the last minion, the first minion, and 'sometimes' the ones behind it are OBVIOUSLY given a huge speed boost, for at least to be, what looks like 2-3 seconds.

I have no idea if this rush is intended, or not, and it gets me killed. While you would think it be nice, for engaging the towers, if one minion or a couple hurry, while Rengar is in the bush, literally my melee minions run to their death randomly and my casters still continue to walk slow. Not engaging the tower together, feeding Rengar some tasty caster minions the melee ones don't even defend, and are the most convenient ones to retaliate counter play to.

Nothing is wrong with minion movement speed or aggro. But what is with this random HUGE movement speed buff to individual minions based on?