What makes a betta swim this way? (I dont think it can be a good thing)

[RayneForhest]

They curve up like a comma... and instead of swimming horizontally... they swim vertical.. and it makes them look like they're trying to use their caudal fins as human feet/legs?

I have 3 that are doing that. At first I wondered if it was a swim bladder issue... (skeptically so, as I've seen some bettas who CLEARLY have a swim bladder issue... and they float differently...and mine doesnt do that)

It almost seems as if the problem is structural...
Is this something they grow out of? Could this be from travel?

[Sakura8]

I don't suppose you could get a video of this? I'm having a hard time picturing it . . .