When I first saw my fish trying to go up the waterfall I too was curious. So I sat and watched for what seemed like hours. The conclusion I came to was that they were playing.
The lip on the edge of my latest waterfall now sits a couple inches above the water level. It's due for a rebuild and I just might make it possible for the fish to 'play' again. I really miss watching the fun.
they are carp, common brown carp swim miles and miles against the current up into smaller ortuaries to spawn this may be a springtime instinctive behavior ( the waterfall being the only current). just a possibility, i very well could be way off base