From a bit ago but still well worth reading: a typology of Ukranian oligarchs and therefore a comprehensive explanation of why Ukraine is a neofeudal state, enlivened by plebiscites and violence. As to how this maps on to the current situation:
The vilest of all the oligarchs bred on family ties was the son of the fourth Ukrainian president, Aleksandr Yanukovich. The eldest son of the president, Aleksandr Yanukovich, embodied the so-called "Family" of the president, which was comparable in certain respects to the Yeltsin family. He was by training a stomatologist but, according to calculations by Forbes, Yanukovich's son owned capital worth half-a-billion dollars. Unofficial calculations suggest, however, that this figure may be six to eight times higher.
Dmitri Fyrtash, also profiled here, told FBI investigators that half shares of all his companies were owned by the Yanukovich clan. Kolomoisky was put under similar pressure. The basic issues behind the formal issues in Ukraine might be that the Yanukoviches just got too greedy.