Generally speaking, foreigners will get secondaried because they simply say too much. If asked why you're here, just say tourism. If you're traveling alone and say you're coming to see a friend, that's an alarm for CBP. Maybe you're coming here to get married or something that would cause you to stay. If you feel the need to say anything other than tourism, say you're coming to see a brother or something. IMHO that's too much info, but saying you're coming in to see a friend is begging for a secondary.

Oh, and when they secondary 90-pound Thai women, they aren't thinking she's a terrorist. They think she's coming here to maybe hook and will staying for a while to do so. Or just coming in to live illegally - it happens all the time. There are a lot of illegal aliens in the country who didn't hop the fence from Mexico. They come in on tourist visas or are from tourist-waiver countries, and the just decide to stay.