Anxiety is quite often irrational, and therefore you will not be able to predict everything that triggers it. I have often been floored at things I thought innocuous that caused an anxious reaction in my kids. And conversely, things I found bothersome or scary had little to no effect on my kids.
Exactly this! What scares me makes my son laugh out loud and what is normally considered acceptable might scare him to the point of tears and panic attacks (a family friendly Disney movie, an age appropriate biography of a famous President and many other things).
In our case, I found that it was best to protect my son from those triggers and eventually, in a year or two, he has been able to handle the same content with as much rationality and calmness as a normal child who might be 2-3 years younger than him. In our case, I am chalking it up to asynchronous development.
So, don't worry, I think that there was no way that you could have known that your DS would respond like that to the exhibit.
