From what I have been told by TSOs, and from my own experience, the following tend to touch off inspections of checked baggage:

1. Large amounts of chocolate (which looks like explosives).
2. Several books placed together (see above).
3. Liquids (other than those packed in your toiletery kit).

I now carry chocolates in my hand luggage. I bought a separate suitcase on my last trip to the States in which I put the 30 books I had bought. I no longer pack bottles of liquids after this mouthwash incident.