Well, the pet shops that feel they must sell every thing to every little snot-nosed whiney kid that comes into the shop imports all these non-native species from south america and southeast asia. Even the aquariums.
Then when that cute little snake or alligator get five feet long, we can't take care of them anymore so release them into the wild.
Someone just caught another Piranha here in a North Carolina river. Yup, another non-native species prohibited from owning and releasing into the wild.