During the spawn, when fertilized Bass eggs are layed into weeds ducks and geese occasionally pick them up with their webbed feet when swimming by. When moving from pond to pond or lake to pond ect.. some of these eggs fall off of the feet leaving a fertilized egg in a new or different body of water, these eggs will hatch and begin a new population or contribute to an already populated body of water.
It can be that theory above or in some cases people will stock these ponds which IS illegal but some people don't know or don't care, this can cause a great concern because sometimes you never know if the fish is invasive or not.