All Ukrainian citizens speak Russian but not all of them speak Ukrainian. The Russian language is considered the standard, much like standard German in Austria/Switzerland or Mandarin Chinese in China (incl. Taiwan and Hong Kong). If during the time of British rule, Hong Kong had declared Chinese as an unwelcome language, it would have been considered an aggression.
At the border of empires (that´s what Ukraine means btw: borderland), people often rebel to create their own state, see the American revolution or the American Civil War. Imagine if in the American Civil War, the South would have tried to join an adversarial military alliance. If you want war within your new state, you do stupid stuff like this (or banning official dominant languages).