FYI, if you replace iruka with say.... hinata, the game won't wait either and will cast at beginning of the fight.
The significance here is that iruka have an interrupt skill, which on auto the game will wait on before spending chakra so that it can be sure that you have chakra to interrupt enemy mystery, only after your (and enemy's) first character have acted, is the auto system reasonably assured that they wont be casting anything, ergo stop waiting and reserving the chakra for the interrupt cast and let you cast whatever it is you would normally.
So as far as 20 chakra first turn castable chars, only iruka, Ao and Neji(GNW too) does this. Any other 20, you would cast either that or your main's mystery immediately.