While impatiently waiting for my new big tank to arrive (oh yes, it's on its way, all 160 cm of it), I have been amusing myself by looking up various fish species and planning what to put in said big tank. I came across this video while looking up info on hillstream loaches.
"Muahaha! Your silly glass boxes can't keep ME inside! FREEDOOOOOOOOM!"
As is often the case with seemingly baffling fish behaviour, it has its roots in the way the fish behave in the wild - in the case of this climbing action, it's how they travel past fast currents. Here's a neat article for those inclined to read more about this behaviour.