What triggers trypophobia?
Plenty of things. Naturally occurring holes -- or shapes that mimic holes -- seem to be the origin of the phobia: honeycombs, coral, acorn holes created by woodpeckers, sandstone structures, garlic, lotus pods, strawberries, condensation, bubbles.
Photo manipulations that show holes in the skin -- often with pods or other objects inside of the holes -- have popped up as a result of the phenomenon, often to "test" people's triggers.
Those who suffer from trypophobia are said to experience goosebumps, a feeling of repulsion, skin-crawling sensations, eyestrain, panic attacks, and nausea when they see these triggering images, either in photographs or daily life.