I think you're confused a bit there MegatopLover. There are two-way travellators the entire length of the D concourse at level 2, sans the duty free area in front of the immigration entrance. There are no one-way only travellators on that entire stretch.

As to trying to traverse the D concourse on level 3, that should not be possible based on my experience. Everytime I go through any security checkpoint, they always ask to see my boarding pass and won't let people in if their gate is not on that concourse. Perhaps though if they didn't check on that occasion or if the gate number wasn't printed on the boarding pass and they didn't realize it wasn't a D gate. If it did happen though, as you say that would have been a good reason for the reported double security check that I've never encountered.

It should be noted that when transiting domestic to international, after you go past the guard you then go up an escalator to level 3. AFAIK, there is no way to stay entirely on level 2. So if the OP was unfamiliar with BKK could have perhaps not realized that you could go back to level 2 and as you suggest went across on level 3, or got to the security checkpoint and was refused and then had to backtrack and up to level 4 or down to level 2.

Good point about perhaps the aircraft arrived at an international gate. I've heard that reported here but never experienced it myself so hadn't considered that possibility.