I was hoping someone else would chime in that knows about this rule, because I don't. I have noticed this type of construction now and then, but didn't know it was that way in every case. Somehow I had the impression it was originally done to make the word/syllables/tones roll off the tongue easier. Several short syllables in a row, with the tone jumping back and forth from high to low, might cause your speech to stumble. I'm probably wrong. I've had Spanish teachers tell me certain words do not follow the rules, or have special rules, so they roll off the tongue easier, and/or are more pleasant to the ear. I would like to know more about the rule in Thai. Here's Google's take on what I'm trying to explain: To "flow off the tongue"—more commonly said as "roll off the tongue" or "trip off the tongue"—means that a word, name, or phrase is very easy, smooth, and pleasant to say. Meaning and Use Easy to say: The sounds fit well together in the mouth. No stumbling: You do not trip or get stuck on hard consonants. Common variants: People also say "roll off the tongue" or "trip off the tongue".
