I have entered the border via GE few times but never been flagged for secondary.

There was no X on the receipt and I was on merry way until customs officer near exit stopped me and continued to ask few questions such as length/ purpose of my trip, my profession etc.

Then he wrote S on the receipt and sent me for CBP baggage check. Where the officer asked same questions, opened and inspected my bag and then we were done.

Just wondering, if this is random at MIA or happens frequently when coming from Brazil (or S America)?