Three of the bonus dailies are constant and the other three are random. I initially thought they changed daily but yeah, they actually seem to change after each battle. If you have the time, you can check back after each defense and maybe get a few extra commendations. I think it is probably intended, given the relatively large number of badges needed for the rewards, and of course you wont always have the bonus dailies available.
Make sure to hand in the ones that change before a battle though, until the next patch you have to wait til they are being offered again to actually turn them in.