Catfish move up into creeks to spawn just like other fish do. Blues especially. It may be mother nature's way of insuring a concentrated food supply for all the baby fish. Lot's of eggs and lots of fry to feed on in a small area plus all the vegetation washed in by the spring rains. By summer's end most of the surviving spawn will have dropped back into the main lake.
From Genesis: "And God promised men that good and obedient wives would be found in all corners of the earth."
Then He made the earth round...and He laughed and laughed and laughed!