How do I know? It's happened to me 3 times. And yesterday, when I made a point of wearing pants to the airport, I saw 2 women in long skirts — there are some really pretty sundresses out there — and both of them got pulled aside for a pat down.
ADDED: I'm guess the government has decided to pat-down all wearers of flowing skirts as a general rule as a way to search all females in traditional Islamic dress. Any complaints about profiling will be easily met.