Someone is going to have to help me out here.
We have a seal structure that for lack of a better term has a weepage and drain tubes to handle normal flow provided that there is a proper pressure supplied in the air buffer cavity.
So for lack of a better analogy then possibly there is some disturbance in the air buffer pressue cavity supply that can happen at higher thrust settings allowing the seal to become ineffective due to the higher gas velocities at or near the air supply point for the air buffer cavity.
Just a wild guess, if so could be an easy fix to the design.
Well, 'fixable' it certainly is... but it may require a bit of thought...
Think of a turbine disk... there will be a pressure drop across the turbine disk as the hot gases flow through it. The overall pressure ratio of a Trent 970 is about 39:1, I believe, so (very simply), that's the pressure drop that is taking place across all the turbine stages. You can't 'seal' the gas passage, so the faces of the disk 'see' the pressure that is in the gas stream. Thus the pressure on the face of the disk will be higher on the upstream side compared to that on the downstream side.
Now think of a bearing chamber that is supporting the shaft that carries that disk. There will be a higher pressure on one side of the bearing chamber compared to the other.
Balancing up all those pressures, removing, or scavenging, the oil from the bearing chambers so that it serves its purpose of heat removal and lubrication, and at the same time ensuring that the pressure in the bearing chamber is (crudely), lower than the pressure outside the bearing chamber so that oil does not flow out past the seals, are all part of the air and oil system design of the engine. And it's an important phase of the development testing.
So, if something has happened to disturb that carefully balanced system, then possibly there is oil leaking where there should not be oil; which is what seems to be reported. That leakage can lead to a fire, and a softened disk, which then maybe lets its blades go, and departs from the engine.