(2) Coal + industrialization + peasants seeking a better life.
The question referred to the Crimea, not the Donbas.
If you wish, you can answer the question: "Can you explain the circumstances which caused the Crimea have a majority Russian-identified population?"
I'd be genuinely curious as to your response.
Crimea is far easier on #2. Crimea has never been majority (or plurality) Ukrainian. Prior to the Russians it was Tatars, but they were exiled after WW2 by Stalin for collaboration with the Nazis. The reason Crimea ended up under Ukrainian control is because in 1954 Khrushchev 'gifted' it to Ukraine to celebrate the 300th anniversary of the Pereiaslav Agreement. That agreement is when the Cossacks that lived in 'the ukraine' (Ukraine translates to something like at the borderlands/frontier) signed a treaty swearing allegiance to Russia. At the time of the gift it was mostly just a token gesture, because Ukraine was just another normal part of the USSR and so basically nothing changed.
[1] - https://en.wikipedia.org/w/index.php?title=Demographics_of_U...
[2] - https://en.wikipedia.org/w/index.php?title=Demographics_of_U...