Russia may impose restrictions or a ban on European airlines using trans-Siberian routes, raising the cost of their flights to Asia, in response to European Union sanctions over Ukraine

A trans-Siberian closure for EU carriers would particularly impact Lufthansa, British Airways and Air France. And that wouldn't come cheap:

losses … could amount to 1 billion euros ($1.3 billion) for three months of restrictions

Flying over Russian airspace is the most convenient route for those airlines flying towards East Asia. A ban specifically targeting EU carriers would clearly put them at a big disadvantage over competition from Asia. 

The alternative route would take longer, using a more southern pathway over China for instance (a flight from LHR to NRT could theoretically also fly towards Anchorage, but that'd be unlikely).

At this stage, the ban is still being discussed between the Ministry of Transport and the Ministry of Foreign Affairs. It follows a new wave of EU sanctions against Russia that apparently forced low-cost Russian carrier Dobrolet to halt its operations and the heavy fines that Ukraine started to levy from any Russian airlines flying to Crimea. 

Aeroflot has also been impacted by a 2011 agreement to reduce the fees EU airlines would pay when using the route. Maybe the move is also a way to renegotiate that deal.