Yanukovych was not a pro-Russian puppet. He and his party did the most to sign the European Association and visa-free travel with Europe. He did this in order to get more votes in the next election. It was kind of a fetish.
At first, no one thought what losses in the economy the signing of the Euro-Association would threaten. The text of the agreement was not even translated into Ukrainian or Russian for a long time. In the end, they calculated and saw what losses and costs it would cost. And Yanukovych decided to postpone and demand big money from Europe for compensation and transfer to European standards.
And then everything came together: the Europeans decided that it would be better not to pay money, but to replace Yanukovych with a more loyal one. The people thought that they were being deprived of their cherished dream - joining the EU (although the European Association did not intend to join the EU). And the Ukrainian oligarchs decided to get rid of Yanukovych, as he prevented them from doing business, and rowed everything for their own interests and those around them.
So the Maidan happened, where many interests coincided.