Following the Russian Revolution of 1917, the military and political situation in Crimea was chaotic like that in much of Russia. During the ensuing Russian Civil War, Crimea changed hands numerous times and was for a time a stronghold of the anti-Bolshevik White Army. It was in Crimea that the White Russians led by General Wrangel made their last stand against Nestor Makhno and the Red Army in 1920. When resistance was crushed, many of the anti-Communist fighters and civilians escaped by ship to Istanbul. A 25-ruble banknote of the Crimean Regional Government

Crimea changed hands several times over the course of the conflict and several political entities were set up on the peninsula. These included:

• Crimean People's Republic -- December 1917-January 1918 -- Crimean Tatar government
• Taurida Soviet Socialist Republic -- 19 March 1918-30 April 1918 -- Bolshevik government
• German and Ukrainian People's Republic occupation -- May 1918-June 1918
• First Crimean Regional Government -- 25 June 1918-25 November 1918 -- German puppet state under Lipka Tatar General Maciej (Suleyman) Sulkiewicz
• Second Crimean Regional Government -- November 1918-April 1919 -- Anti-Bolshevik government under Crimean Karaite former Kadet member Solomon Krym
• Crimean Socialist Soviet Republic -- 2 April 1919-June 1919 -- Bolshevik government
• South Russian Government -- February 1920-April 1920 -- Government of White movement's General Anton Denikin
• Government of South Russia -- April 1920 (officially, 16 August 1920)-16 November 1920 -- Government of White movement's General Pyotr Wrangel
• Bolshevik Revolutionary committee government -- November 1920-18 October 1921 -- Bolshevik government under Béla Kun (until 20 February 1921), then Mikhail Poliakov
• Crimean Autonomous Socialist Soviet Republic -- 18 October 1921-30 June 1945 -- Autonomous republic of the RSFSR in the Soviet Union

At the end of WWII, the Crimean Tatars (a Turkic people of Muslim tradition) were massively deported to Central Asia on suspicion of collaboration with the German occupiers.

Following a link on that Wikipedia page leads to a 1954 article on a pro-Tatar site:

Transfer of the Crimea to the Ukraine

In the light of these facts, the transfer of the Crimea to the Ukraine takes on the significance of a carefully considered political step. The transfer of the Crimea to the Ukraine is in the interpretation of the Communist Party a gift of the "elder brother" to the "younger brother" on the occasion of the tricentennial of the unification of Russia and the Ukraine, as if to demonstrate the solicitude of the central government and its desire to meet the Ukrainian people halfway, at the same time reducing its gravitation toward independence from the Kremlin.

This transfer reveals the long term policy. The Ukraine, as the largest republic outside of the RSFSR, is quite understandably the republic with local sentiments which all the other republics listen to. It is the center in which, as it were, all the republics are united in their national aspirations. The Central Committee of the KPSU had in mind, as well, the idea of weakening the significance of the Ukraine as such a center when it ordered the Supreme Soviet to issue this decree. In the first place, the Ukraine, having received the Crimea, an area which in fact belongs to the Crimean Tatars, at the same time makes itself an empire to a certain degree, for now it possesses lands without justification based on ethnographic principles. Therefore, it is the Ukraine and not the RSFSR which turns up as a party to the dispute over the lands of the Crimean Tatars. This places all the republics of Central Asia--the whole Moslem world of the USSR--in opposition to the Ukraine.

Thus, nearly twenty-five million members of the USSR's Moslem world will no longer look on the Ukrainian SSR as their ally in the struggle against the Kremlin's imperialism, but on the contrary will look upon it as a republic with imperialist tendencies which, by virtue of these tendencies, should become an ally of the Kremlin.

I repeat, it's a pro-Tatar site that offers this explanation.