It was a violation of the 1994 Budapest Memorandum. On December 5th 1994, the heads of USA, the Russian Federation and great Britain all met in Budapest, and agreed that, if Ukraine disarmed itself of its large nuclear arsenal, then its boundaries would be assured.
Russia violated this treaty, using the Euromaidan protest as an excuse, their argument was that, because there had been a ‘revolution’ in Kyiv, then Ukraine had changed and become a ‘new state’. This was stretching events to suit the Kremlin aim of grabbing territory whenever an opportunity comes along. Ukraine had not changed dramatically, but was rebelling against corruption. In Russia, the narrative was that is was a US led coup. It’s impossible to say how much of an influence the USA had in the protests. It wouldn’t surprise me, if, in years to come, it emerges that Russia and USA did a deal over Ukraine, before Euromaidan even happened.
The referendum was conducted in some haste – within 3 weeks of the annexation of Crimea, the votes had been cast. But, how can a referendum be held when a territory is under military occupation, with Russian soldiers patrolling, with heavy armour and artillery, and with the gun smoke of unrest in the air?