Could have been chasing missiles, though this is improbable given all reports on the S300's have concluded 100% effectiveness. Another potential could be someone in Ukraine being overzealous and trying to trigger a false-flag to force NATO into the action. Ukraine's president has been making continuous efforts to pull NATO into their war and may be getting desperate especially given they are going into deep winter and both sides will have to make significant strategic changes. It would not be the first time a major war started from a false flag. Thankfully either way cooler heads prevailed this time.