Question: What do you call it when your mind starts examining things automatically because of being exposed to these things in the past. This is not intentional thinking nor wandering but rather an automatic response in your mind

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> John has quiet time at home and wants to just relax, but he can't stop his mind ... the events of the day.
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Using *Reflect and Review* make the thinking seem voluntary when I am trying to convey the involuntary aspect of it.

Answer: 
Consider: Ruminate.

"...he can't stop his mind from ruminating over the events of the day."

The definitions I've found ("Think deeply about a subject or question over a period of time") don't capture how this word is usually used. The definitions I've found miss the connotation that rumination is not intentional, and in fact happens in an obsessive way despite attempts to think of something else. 

This article gives a better definition, and shows how the word is used in context you describe in your example: <http://psychcentral.com/blog/archives/2011/01/20/why-ruminating-is-unhealthy-and-how-to-stop/>

"Ruminants" are a class of animals that chew their cud. The psychological meaning comes from the metaphorical similarity, where you play your thoughts over and over in the same way ruminants chew their cud over and over. See the "Other Uses" section of <https://en.wikipedia.org/wiki/Ruminant>.
This isn't a single word, but I'd probably write that John "can't stop his mind *from running over* the events of the day." "Going over" also works, though it sounds slightly more voluntary to me than "running over."

Based on the tone of the example, I also thought of "perseverate," a technical term from psychology meaning to get involuntarily stuck on the same compulsive thought pattern or behavior (source: New Oxford American Dictionary and my experience working at a nonprofit for teens and adults on the autism spectrum, where we use the word daily). "Perseverate" probably isn't a good choice for you, however, because a) it's jargon, b) it applies equally to behavior as well as to thoughts, and c) it applies only to repetitive, compulsive thoughts. Also, while perseverative thinking is often associated with anxiety, it doesn't have to be, and anxiety seemed central to what you were looking for.