Faced with cuts to state pensions, Ukraine started using gas from the pipeline which connects Russia to Western Europe, without paying for it. That understandably annoyed Russia (that’s not a justification for war!), who couldn’t turn off gas to Ukraine without also turning it off for their main customers in Western Europe.
These events seemed to have kicked off the norstream pipeline (legal) and invasion of Crimea (illegal).
Here is a contemporary article less than a year before the Crimean invasion: https://www.theguardian.com/world/2013/oct/29/russia-ukraine...
See also: https://en.wikipedia.org/wiki/Kharkiv_Pact#Effects