Once inside, they instinctively swim against the strong current of water rushing downward. That means they end up moving backward down the spiral, eventually getting pushed out the bottom of the helix into the river.

This method has lead to better survival rates than when the fish were spilled over the top of the dam.

Salmon like to go downstream backwards

https://ecology.wa.gov/blog/august-2024/salmon-restoration-project-helps-strengthen-sockeye-population-in-the-yakima-basin

I guess they try to escape the current

I dont know, the video was too long