It’s probably the comfortable mouth shape from their native tongue. If you ever try to speak a language fluently you’ll notice that your mouth is not used to these shapes and fatigue will set in eventually.
I’ve noticed this myself when I was in America. After using the language 24/7 my mouth would start to cramp up, and falling back to my native mouth shape made the cramps less. I pride myself on my English pronunciation, and cringe when I hear other people from my country speak English, but I understand where it comes from.