Answer is very simple.
Who are oligarchs?
They are, as we know, big capitalists - small group of men, controlling big amounts manufacturing, and money.
Why do they appear on communist country fall?
Because they ARE a part of capitalists, free-market world.
Why do they appear bringing corruption and bribes?
Because they appears on a country dramatic and sometimes bloody changes - and they are doing money on a country, or even empire, fall. And those dark times always brings corruption and criminal uprising.
What is their differnce from bureaucrats?
Imagine situation: oligarch owns a factory in such dark times for country. He has two ways: upgrade factory, pay salaries, and so on, OR sell factory to the scrap, have some money from it and leave the country. Do you think, where people should work, if there is no more factory, and this is a mono-city - city around big factory which is now closed? Yes. Nowhere. Bureacrat, instead of it, cannot close factory, because he DO NOT owns it. He even cannot (or it is hard to) leave the country, especially with money. He is just manager. That brings difficulties in changing - because it is always more simple to leave things going, but also bring some protection from closing factory and leaving people unlaboured.
Soviet union falls into it, changed and transformed into modern Russia, which controls oligarchs in some way. China had left in a step behind, and change communism system in a some way - they have oligarchs, but they didn't have 'wild capitalism' period.