Does Runic Carvings trigger it? That's another card that HAS an Overload component, but it's a choice and not a default aspect of the card. Which, if that's how they've programmed it, seems a bit silly. Why not just have it be "When you Overload" or something.
It helps to clarify the difference between something like The Amazing Reno - where your side of the board can do something that causes you to overload, but it doesn't trigger effects for Unbound.