They've has been doing those spot checks after Customs for at least ten years, mostly looking for excess duty free.

Wife arrived yesterday morning in a half empty TG917 from LHR. Telephoned me and said that after clearing customs got stopped again by the military and bags checked. She said many Thais were being stopped and farangs were being ignored. I kept a diplomatic silence and of course she couldn't see me smiling.. This is the first time in 30 years she has been stopped again I kept a diplomatic silence. She also said the food was terrible. but she had 3 seats to spread out on and had a good sleep.

As has been mentioned, they are Excise, and they are looking for alcohol and tobacco. In the past, I have gone through the Red Channel with expensive wine and declared it. Customs encourage you to throw it away and go to the Green Channel, but will accept customs duty and VAT if you insist. You get a receipt with your name and passport number, but they cannot collect excise duty because it is illegal to import commercial quantities (i.e. over one litre) unless you are a registered Excise Tax payer.

The situation is utterly ridiculous, and I have stopped doing this because it you do get stopped by Excise, you will have the goods confiscated and you may be fined, even though you paid (some) taxes.