Why was I aggressively questioned at immigration while returning to Thailand on a METV?

Did a bounce yesterday. DM to KL return. I'm on a METV. On return TO DM, got taken aside and given a grilling. The guy at the booth was very aggressive. Asked me what I do here in Thailand. I told him rightfully, that I'm a tourist on holiday. He implied that I was working here. He sent me to an office where I was given the 3rd degree. Luckily I had plenty of paperwork. I got in, but walked away.. 1)disgusted at the way I was treated and 2)none the wiser as to why they grilled me.