People brought them to the Pacific Northwest during the Great Depression and farmed them - frog legs were a delicacy. There were bullfrog farms on Cottage Lake, Paradise Lake, and other ponds and lakes around the county. When people were done farming, they just opened the pens and set the frogs free. Some people dug new ponds especially for raising bullfrogs. Even recently, people have shared their eggs with their neighbors to spread bullfrogs to new habitats, because they didn't know the damage the bullfrogs could do.
Bullfrogs are a problem because they have voracious appetites.
They hunt by waiting for their prey to get close enough that they can grab it - and they'll eat anything they can fit in their very large mouths. They've been known to eat insects, other frogs, tadpoles, salamanders, and even birds as large as robins, and ducklings. Our native amphibians, who aren't as prolific as the giant bullfrogs, often can't breed fast enough to keep up with the large appetite of the bullfrogs. May. Compared to native frogs, bullfrogs are incredibly prolific, and are quite commonly seen throughout King County. As the Washington State law is written today, bullfrogs are a game species.