Here's my guess on things that could lead to a patdown, starting with most likely:

• Bulky clothing, skirt, hat, etc. TSO decides is a risk
• Random explosives swab tests positive
• WTMD alarms and TSO decides you can't try again
• Checkpoint's schedule of random things to do hits when you get to the front of the line and it specifies patdown
• WTMD randomizer decides it's time to pat someone down
• WTMD alarms several times and TSO gives up and orders patdown
• Something "suspicious" in your luggage and TSO overreacts
• You forget/lose ID
• Boarding pass gets SSSS
• For some reason you have to go through the same checkpoint several times (smoke break? left something in car? etc.) and it raises suspicions
• TSA decides to add a gate screening that involves patdowns for some/all
• BDO decides you are threatening
• Some "security breach" happens, severe enough to dump the terminal and pat down some/all on return
• Some general threat warning is spread by TSA - perhaps they decide diapers are a threat that day?
• For no discernible reason, TSO or supervisor decides to add a "layer" to security by deciding to pat down some/all travelers

Here's a few risks you might not have considered:

• Between the time you buy the ticket and when you travel, one of the airports gets a bodyscanner
• Weather/mechanical issue strands you overnight in a connection city and you have to clear their security
• Flight is diverted to an airport with a bodyscanner and you have to leave for a hotel overnight

And there's still the general risks you face whenever dealing with TSA (theft, damage to property, etc.)...

There's really no way to guarantee you won't be accosted. How much risk are you willing to take?