I've been referred for full secondary inspections six times and agriculture onse 2-3 times.

For full secondary inspections, they have always taken my passport and always searched my bags.

For agriculture, it's more low key, always a bag x-ray, and then I can't remember if there was a passport check or further search.

If it were about taxes, they would have asked you about what you do fora living and tax related issues. It sounds like a rather routine agriculture inspection, the least intrusive secondary inspection yo can get. They may have taken your passport mainly to check if you have a history of prior agricultural violations.

I wouldn't worry about this at all. It's like complaining over and over "A cop pulled me over for speeding and checked my license and regitsration and gave it back and wished me a good day with a warning and I want to do why he didn't as for my passenger's ID and why he didn't pull over other cars going the same speed."

You'll never know if it was really random, it could have just been a hunch based on your nervous or hesitant demeanor with the primary officer, not making eye contact, you history of frequent trips to Mexico, your girlfriend being suspiciously way better looking than you, who knows. They can send you to secondary for just about anything, and they often won't tell you why.

I do recommend making a FOIA request for the records, there may be notes that shed a little light on it. Very little light. Please update us if you find out anything else.