The density of various items can trigger a search. I used to get it all the time when flying with big pieces of glass for my camera; the flourite glass, according to several TSOs, was tough to either determine what it was or what was around it and necessitated the hand check.

Oddly enough, that never happens any more. I tend to think that the x-ray operators are very distracted these days, trying to catch everyone's lip balm and water, and miss a lot of things they don't give attention to.