I look at it like this you put them in small tank 120'' x 4'x 4' and that's not small to us as keepers but in the wild I bet they swim up and down back and fourth all day throught the river systems. So if you see it from there point of view I would yawn a lot also in a tank with really no where to go. As a matter of fact I yawn all day long from being in my 4 x 4 cubilce.
low oxygen levels can cuase them to yawn alot.
if its not,it could be that theyre hungry or have just eaten.
mine yawn alot when they get very hungry and they also yawn alot after a big meal.