The most report explanation is that UKR fired an air defense missile to intercept the RUS missile that was attacking something not far from the border. The air defense missile (made in USSR, I presume) did not catch the other one and then failed to explode in the air as it supposed to.
UKR are currently worried that the West is interested in a “peace” settlement too soon, letting RUS keep occupied areas. Achasheverosh advisors are now saying “up to UKR to decide when to negotiate” while applying pressure behind the scenes (these are same chachamim who were so successful in AFG and then preventing RUS start of war with “stern warnings of future sanctions”).
So, UKR are very sensitive that people in US and Europe will stop supporting them, falling for RUS propaganda, as we see some here.