Re: Muay Thai
I think it's just adaptation to their environment and being exposed to new stuff. Dutch kickboxers first brought the heavy punching style to Thailand. They knew they couldn't win on points because of bias judges and went for the K.O. Perfect example is Ramon Dekkers.
Then you have fighters like Buakaw Por Pramuk who is a traditional Thaiboxer who focused 95% on kicks who added a boxing coach to his team and was suddenly knocking his opponents out with right hooks and slipping/dodging punches like a pro.
Knockouts are never guaranteed and they score things differently so the majority of fighters tend to be stronger with their legs.
They aren't changing up their stance as it would leave them too exposed to low kicks. From what I've seen they are moving/dodging alot more like western boxers. If the majority of your opponents are strong kickers then you're probably not going to be fantastic at defending against someone with powerful hands and that's where boxing training would come into play.