It depends!
Mine sometimes freak me out by floating at unexpected times, and one of mine just seems to like floating with the very top of his head out of the water (not his mouth/nose, just his head) for hours.
Have you checked the water parameters? If its a new tank, was it
cycled when the axies went in, or was everything new?
If you have a testing kit, now would be a great time to check that everything is safe for them - if you need more info on this just ask!
Also, do you have gravel, or were they on gravel at the pet store? They can sometimes get, errrm, gassy, and float because of that. If they have been on gravel, there is separate advice you may need to follow to make sure any swallowed stones pass through ok!
But, as I say - it seems a lot of axies do it just for the sheer fun of it, so it's probably nothing! In fact - I think I posted an almost identical post to this when I first joined!!
Hope that helps some
(ps: sorry Shannon, looks like I overlapped with you!)