The more legitimate reasons are to resolve an alarm on one of the security measures. There's also watch list screening, as there are certain "names" (hey, TSA doesn't put people on lists ) that are going to get the business every time they fly, no matter what. There was always the dreaded SSSS that showed up on tickets that guaranteed you hit the lottery for a secondary. It's possible he may share a name with someone on the list. There are common names on there like Robert Johnson, David Nelson, etc, so everyone with those names can possibly get hassled. SecureFlight was supposed to fix that, but it doesn't work very well.

Unfortunately, there are a lot of BS reasons why it could happen. TSA often uses "continuous screening" as cover all to justify a secondary.

In some cases, TSA picks on people who look like they're more likely to comply and not hassle them about it. In others, a screener may be attracted to the pax and wants to get a "closer" look. We've seen this with younger, attractive getting flagged for body scans and secondary pat downs, but it could happen with anyone that tickles a particular screener's fancy.

There are retaliatory secondaries if you challenge a screener on policy or a rule. Many of us have gotten those.

If a SPOTnik thinks you look funny, or doesn't like your answers, you could be sent to secondary.

If a person has a medical device or other disability, that can cause alternative methods to be used.

Lots of reasons, many are garbage, and unfortunately, unless you notice something that he always does, or if he consistently acts in a certain way at a checkpoint, it can be really hard to tell.

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On another note, while writing this, I just thought we could start a thread much like Jeff Foxworthy's "You might be a Redneck." We can call it " ... Then you might be secondaried."