Why do humans suffer anxiety when they view "Trypophobia trigger images"?
Trypophobia is not a recognised specific anxiety disorder (Washington Post). It is worth mentioning that anyone can have a phobia to anything, this is merely a question of whether many people associate these spatial patterns with anxiety. Nevertheless, the response of individuals to these images can be quantified (Le et al., 2015). Ultimately the findings show that a response of trypophobia is not correlative with anxiety. Note that here we are discussing anxiety in a phobia response test. Typically anxiety manifests as sweating, dizziness, headaches, racing heartbeats, nausea, fidgeting, uncontrollable crying or laughing and drumming on a desk. This is not merely feeling uncomfortable.

One hypothesis was that these images had irregular spatial patterns that cause revulsion. A study found that in nature some animals and plants may use this patterning as a warning mechanism and that it is associated with poisonous animals  (spatial pattern quantification of 10 poisonous animals versus 10 control animals p=0.03), and indeed spiders were among those that use irregular patterns (Cole &amp; Wilkins, 2013). Note that this hypothesis was presented in a psychology journal so the evolutionary mechanisms remain, in my opinion, not fully explored and scrutinised. 

Hover over the below yellow box to view a lotus seed head, which has typical irregular spatial patterning presented in the 2013 study. 

This image is often reported as inducing trypophobia.


   


Answer: In summary, humans do not reliably feel anxious when viewing these images. It also remains unclear why some people do get anxious or uncomfortable when viewing these images. It is perhaps to do with an aversion to some potentially harmful animals, but evidence remains scarce.