Some Minions Randomly Freeze in Place Until Enemy Wave Aggros

I have noticed this on many occasions. Usually it's a siege minion. (Both normal and super)

After the minion does its duty, it decides to just slack off on the job and hang around in one spot until provoked by other enemy minions.

My favorite was when the enemy team (purple) had super minions. My team had been aced and their wave cleared ours. Nexus turrets take out the wave, but what's this? The siege minion was in the middle of our base just standing there... waiting to take on my whole team.