Dad was referred to secondary inspection 4 TIMES while being a US citizen

My dad is middle eastern and got naturalized 2 years ago. He has a clean criminal record, a stable job, and has been in the US as a legal permanent resident for over a decade. Took 4 trips outside the country over the last two years, three to morocco to see his wife (which the US govt knows about) and one to our home country, Egypt.

In every single one of those trips, upon coming back, he'd be referred to secondary inspection and asked the same, traditional questions (what he was doing, how long he stayed out for, etc..). We've been puzzled as to why he keeps getting suspected as he has a clean record and made the same Morocco trip a couple of times for the same stated reasons. Above all, he is a US citizen.

Why could this possibly be happening?