Ahh, dirty rushing....
One of my best buds lost out on being in a sorority cause of dirty rushing, so the whole subject really @#$%s me off.
Promising someone a bid is the most common type of dirty rushing. Other things are:
-spreading rumours about other sororities
-cornering a rushee and pressuring her to join
-socializing with rushees outside of rush (if you didn't know them previous to rush)
-buying things for rushees
Why would a group do this? The only reason I can think of is to try to cut their losses. They may not be sure if they want her, but they don't want her to go elsewhere in case they need her to fill quota/total. 75% of the time they don't need her and she gets dumped on her head.
Also, I think sometimes newer members may not be totally informed and dirty rush without knowing it. In my friend's case, she was promised a bid by our RA (!) who had only been a sister for a semester. She might have really thought she could get my friend in, I don't know. But obviously, ignorance is NO excuse.