The explanation I have heard and believe from the evidence I have seen,is;that anadromous fish make their main migration moves during the dark hours. Towards daylight ,having negociated some difficult water,they will stop in the first good holding pool. The more agressive the species,with chinook at the top of the agression list,they will circle the pool driving out any less agressive species including any foreign objects like spoons hot shots and spinners.
A somewhat similar period of activity occurs just before dark in the evening.Fish will circle the pool gettibg ready to move if they encounter eggs or worms they will grab them as a reflex knowing they will need all their energy for the next night`s migration.