[Maybe Russia was about done with those “mercenaries” and thought letting the US kill them was a good way to get out of making the final payment on their contract?]
Unlikely. They might have thought that the mild US reaction to harsh Turkish rhetoric meant that they could do whatever they wanted. Besides, losing people like that makes it hard to recruit quality people in the future. The strong US reaction also means that the Turks are under no illusion as to what the US response might be to a Turkish attack on US troops in Syria. I don’t think the Turkish public will think better of Erdogan if he gets hundreds of Turks killed in an attack on American positions.