When services are diverted,
@yorkie is correct that the default position is that they must fit around any existing services. Clearly, when you have a flow as significant as Manchester-Leeds/York etc. being diverted, that means that many people are being inconvenienced for the sake of a much smaller number who are on the existing services, whose paths are seen as inviolable.
One can only hope that bringing TPE under the umbrella of the OLR might bring some synergies and better cooperation with Northern. Whilst there has been a significant amount of planning involved to ensure that diverted TPE services work around Northern ones, it has very much been done under terms of reference that reflect a parochial attitude of not inconveniencing existing Calder Valley passengers, rather than looking at the bigger picture and minimising overall impact to passengers.
In a sensible, well-integrated railway it would be recognised that if an extra circuit was required in the Blackburn service to allow the diverted TPE service to get a better path - which in turn might save a circuit there - this would be a sensible compromise.
Sadly, that's not the world we live in and instead we have a scenario where taxpayer money is being poured down the drain by having different TOCs essentially engaging in a turf war. On top of it all you have FOCs receiving taxpayer subsidy to occupy the best paths, to (occasionally) transport "environmentally friendly" wood pellets across the Pennines to be burned close to an east coast port. You really couldn't make it up if you tried!