It's a growing trend of voyeurism called "up-skirting"   where sickos photograph or film you, without your knowledge--We read about it in Newsweek magazine.
Typically the assailant is seeking out women wearing skirts and secretly photographing them from underneath   that's why it's called "up-skirting". Then they sell these pictures and videos to websites. It's not only happening out in public but in locker rooms, dressing rooms and public bathrooms too. There was a recent case of a man caught doing this at a music festival in Seattle--But he was only sentenced to 38 days in jail because under the law, the woman being photographed was out in public so she shouldn't expect privacy.
Since that case, the law has been re-written to make the space under a person's clothes private, even if they are out in public. And 44 states do have some kind of law that makes video voyeurism a crime.
But these laws are hard to enforce because they're worded very loosely to preserve the rights of people videotaping legitimately. There was a recent federal law passed making it a crime to secretly videotape or photograph someone while they're on federal property   but that doesn't protect the majority of people out walking around or going to the gym.
New laws are being written so that pictures can't be posted on the internet without the subject's permission. But they're not in place yet--So be aware of your surroundings no matter where you are. It's a scary world we're living in.