I've never had that trait, but I suspect it is caused by a resentful feeling of being treated like an infant.
My girlfriend does this all the time. She sees puppies and shit like that on Reddit, and always does this like "growl" and says how she wants to "squeeze the shit out of them." It's not like she really wants to squeeze them and kill them, and it's not actually anger.
It's just a really intense feeling of joy and wanting to take care of them and play with them. If there's any true aggression there, I feel like it's a result of being protective. It's probably a very biological reaction to seeing a small cute creature or person who needs care.
It's like a seething rage of joy.