He was transporting a customer's vehicle from his garage (repair shop?) In Surat Thani to Bangkok. Said vehicle was on a trailer towed by the complainant in his own unidentified vehicle. Thats a long journey.
Likely, the police stopped him knowing that the trailer would not not meet traffic regulation (lights, registration etc) and was ripe for a 'ticket'
All his documents were in order.......this I doubt.
He set himself up. If the Chumphon checkpoint had not stopped him the later Hua Hin Highway 4 checkpoint would have.