It's concerning due to Kazakhstan's size and geographical location. As someone else pointed out, it borders both China and Russia. It's a huge concern for Russia because they are extremely invested in Ukraine at the moment and do not need this instability on their south border distracting them. China has similar concerns, but to a lesser extent. I'm not convinced the US had anything to do with this yet, but it would not surprise me if that eventually turned out to be the case.
Kazakhstan is old-school Soviet, which means Putin has not had to worry about it. The country has served the same buffer role for Russia's southern border as Ukraine has for its western border. But Russia screwed up by issuing their warning against outside interference. They have revealed their concern (ie weakness) to the world, and there are actors who will attempt to take advantage of that. Russia telling the US to NOT do something is a guarantee of the US doing it anyway. If they had just acted unconcerned and said nothing, it would have played out much better for them. Now, the US will be tempted to meddle in affairs that do not concern them and present Putin with a "two-front war" scenario which they might think would offer Ukraine some protection (it won't). Neither Russia nor China would react very well to that kind of development, and I think it would result in strengthening the existing partnership between them.
For the moment, however, I think this is merely a hiccup in the region's geopolitics. We should keep an eye on the situation, but I don't foresee it developing into a major concern.
