Somehow, I get the feeling that Robert Jordan intended for Siuan to have done something far more damaging or dangerous that would draw Egwene's wrath, but it may have only been in the outline as "Egwene is angry b/c of how Siuan rescued her", and it may have left BS in the position where he had to think of something that would fill in the blanks. It was quite weird, for example, that after leading the offensive, Egwene was just left to lie down unguarded. At the very least, you would have expected her Circle of Novices and Accepted to rescue and protect her while she slept. It would have been different if Siuan had rescued Egwene by knocking some Tower Aes Sedai unconscious (either by doing it herself or by having Gawyn and Bryne knock some out) and then use the angreal to escape, it would have been more than enough of a reason to have Egwene be angered.