Because it gives him an opportunity to increase Russia’s sphere of influence (whether by exerting soft power or outright annexation).
Did you ever consider it might be an attack on his sphere of influence? Putin's not the only bad actor in the world.
Bingo!
And who was getting really friendly with KZ lately? Yes, yes!
He doesn't want to annex Kazakhstan, who needs this trouble? He wants it to be within his sphere of influence, but deal with their own problems by themselves, preferably in a way that doesn't give people of Russia any ideas about freedoms or democracy. So far it worked pretty well, but now something went wrong...
The left wing mob (let's face it, HN) blindly hates on Putin. There's probably more to the story (he may be trying to disrupt their oil export deals), but he's definitely concerned about the outcome.