Not aware of a single cause- and rarely are there single causes. This article
Russian pilots of 'bear bomber' plane die after crash | World | News | Daily Express
includes: "but TASS news agency cited sources saying a failure of the plane's fuel valves led to the simultaneous failure of all four engines."
If you are asking about speculation on contributory causes they could include a host of items:
- Aging systems
- Maintenence
- Funding
- proficiency- crews, maintainence, support teams
- An uptick, or surge in flying. There does seem to be a surge.
Most likely a combination of several factors. Pushing aircraft and crews that have not been pushed for years; perhaps deferred maintainence; shortage of parts/funding.....could be a real recipe for problems.