well its hard to explain. I just know out the 10 batches or so of fry I have seen, my batch that died like that, they didnt swim much, they hid, whcih is normal, but some swimming around is normal too. my premature babies would hide way down in the rocks and just stay there, eventually hours later move only to completely hide again. They didnt "swim" either, they just gave one or two tail bursts of speed.
With all my other fry I notice them do alot of swim hovering in one spot, and of corse they hide, espeically if a big fishy comes close..... thats about as good as I can explain it..