You are right. It was Suvarnabumi (say it with me: swan-a-boom). They use the bus because they mix up international and domestic passengers on the same flights. That is, if a passenger checks in at Phuket for an international flight with a domestic segment, when the passengers disembark from the plane at BKK for their next segment, airport staff sort them onto different buses. And the same kind of thing happens when you're embarking at BKK. The planes you were on most likely came in from international destinations and parked at the international side of the terminal to disembark passengers who needed to get through immigration. Domestic passengers can't go to that side of the terminal, so they put them on buses and shuttle them to the plane for the domestic flight.
I agree it's bizarre. But that's the solution they've found for mixed flights and mixed use planes. It's easier for them to have passengers move than it is for them to repark planes.
FWIW, I don't think it's any worse than the mess it is getting from domestic terminals at LAX to the Bradley international terminal, or from getting between distant terminals at JFK hauling bags onto buses with tiny doors. I go through this all the time (3 or 4 times a year).
