In principle, this should only happen if you are wearing "loose clothes" (even an ordinary skirt or shirt can be considered too loose) or if you alarm the metal detector. You can also be chosen "at random".

Basically, if someone wants to grope you or watch you being groped, they can find a reason -- loose clothes seems to cover pretty much anything.

So if they want to watch the 'girl-on-girl' thing with some pretty passenger and/or junior female staffer, or don't like persons of color, or want to 'take down' some uppity business woman type, or have to grab a dumpy white middle age guy every so often, so they can pretend not to be doing the above...well then you'll get the grope. (There is some genuinely random selection.)

Even wearing tight clothes doesn't really help, unless perhaps you approach the 'not safe for work' level. I was once told I was being groped because my shirt was too baggy -- eddiebauer.com sold it (accurately) as a "fitted, close to the body" style.

Of course, not all staff would behave inappropriately, but passengers are completely vulnerable to a dangerous minority. TSA frontline staff have low hiring criteria, undergo only cursory background checks, receive only a couple of weeks of training, and have high turnover. The quality of frontline supervision can be correspondingly weak.