From WENN.com

The actor was queuing at Los Angeles International Airport to catch a flight to South Africa when he was singled out for a search by officials.

He had decided to wear loose-fitting clothing for the journey and did not have a belt holding his pants up - and he was left red-faced when they fell down, exposing his underwear.

Shatner says, ""It was awful to have people looking at me with my pants down, probably the most embarrassing thing that's ever happened to me... It was a long flight so I wanted to wear loose clothing because I didn't want anything to bind me.""