| I can't confirm this myself having never been to a rock concert, but I heard plenty of stories from aging rockers on VH1. They talked about the female groupie who would throw their undergarments on stage at rock concerts, to get backstage, to perform sex acts with the band after the show. This of course depended entirely on the popularity (read that good looks) of the band performing.
Nowadays, women do this when they wish drop those not-so-subtle hints to a man or another woman, that she wants to have sex with him or her. Depending on the woman, this may or may not have been a deliberate act. Some women are brody by nature and generally have no problem with their aim even after guzzling a couple of drinks. Sometimes they have to throw them hard because, some men are very dense.
Other women need to have one or quite a few drinks of confidence before attempting to lob their lingerie at someone. For these women, there is a greater chance for them to miss the desired target when going out with a bunch of friends. Also by going to a popular night club or an after hours hot spot, the number of mistakes could increase exponentially for these shy women.
Finally there are the women who throw their lacy unmentionables out there when they really didn't mean to. Some women are airheads and are genuinely unaware that they had thrown some of their silky lingerie towards anyone in particular. They often hurl their bloomers towards the gay guy they go shopping with often, like Alicia Silverstone in Clueless. These women don't normally throw their panties, they usually drop them, forget where they last had them, or give them away by accident. | 
