The speculation is that Lionfish were transported from the Pacific by pet owners. When the pets became unwanted, they were dumped into the Atlantic ocean, and their rapid breeding rate started creating serious problems.
The chief concern is that, in the Atlantic ocean (the problem reaches much further than the Bahamas), these fish have no natural predators. That means there is nothing to limit the number of Lionfish that could possibly exist.
What's worse is that they are poisonous, ill-tempered and will eat literally anything that is small enough to fit in their mouths.
This is putting serious strain on the ecosystem of the Atlantic ocean, and scientists aren't sure how to stop it.