Because he can. We have moved assets such as anti-missle devices to the border. We are using Iran as an excuse, but they could also be used against Russia.
The build up has been making Russia nervous. I imagine that some of the hardliners at the Kremlin would like to initiate a first strike or something to restore the balance of power to them.
I expect that Russia would like to have those former Russian satellite states back as a buffer between them and Europe.
So on the one hand NATO is too weak to matter. On the other hand, a handful of anti-ballistic missiles designed to knockdown incoming missiles (from Putin’s allies in Iran, by the way) are reasonably seen as a grave threat to Mother Russia who, mind you, has tens of thousands of missiles capable of destroying the USA and all of Europe. And, then, you conclude a Russian first strike might be necessary to restore the balance of power. Is there any limit to the contradictions of Putin apologists? Obama, thankfully, will be gone in days. Putin’s threats to peace and security will not end then. I look forward when patriotism and conservatism mean defending freedom-loving countries and the USA over the real globalist threat, Putin.