Lebrok, your point is sensible. But perhaps metals working knowledge (weapons) could have played a role too? Bronze seem late enough East the Altay, but Iron there seems not very later than elsewhere; and Iron seems the starter of East Asian/Altay first moves. Maybe it doesn't prove too much things concerning culture but the first appearence of strong East-Asian demic imput East the Caspian (Kazakhstan) began at Iron Age, if what I red is true.
Personally I don't think Turks were the first horses tamers but...? It's true these regions were not my first place of interest. I have to learn a bit.