Math - Yanukovich was elected in 2010 on the back of promises to create a visa-free regime with Europe & negotiate an EU trade deal. This was a key plank of his success in the split regions.
Unsurprisingly, his voters were annoyed when he backtracked on this and drew them closer to Russia - as he was going against his campaign promises. He was in fairness trying to get a better oil & gas deal which Ukraine badly needed. But Ukrainians have long looked towards the EU as a way of forcing democratic & anti-corrupt change.
They were also pissed off because under Yanukovich, press freedom was curtailed and there were even more dodgy deals than under Timoshenko, which is really saying something considering how corrupt she was.
Yanukovich had even started losing support from his power base - I was back in Donetsk in 2011 and a lot of Donbassians were starting to look to Tigipko (I am not at all saying that everyone in Donetsk supported Western Ukrainian politicians, just that Yanukovich was losing support in a region that had backed him by a margin of 99-1 - mostly democratically). Visa Free regime with Europe was hugely popular across Ukraine not just in the west. In fact I remember I had a debate with some of my Ukrainian friends along the lines of "you're allowed 90 days of visa free travel in Ukraine, it's not fair!!!!".
The scene was set for a huge display of democratic force, and Yanukovich could have held it together had he not been so scared of Putin and ran off to Rostov-on-Don.
Donetsk held huge pro-Ukrainian demonstrations & a few pro-Russian ones too. The situation was OK...until Russian tanks started rolling in and Russian soldiers started posting selfies in Lenin square (Lenin square is just super - there was/is a McDonald's so you can both in shot).
As I've said before, I think Putin truly believed (as most Russians do) that Ukraine isn't a "real" country; and that Russian speaking Ukrainians would openly welcome Russian forces on the ground. I think he's been disillusioned of that and has little appetite for open war, just wants a constant low level conflict to deprive the new Ukrainian state of the industry of the East.