It isnt just on the Oxford to Reading section that odd things happen with the Cross Country Trains although I've not noticed the early departures personally but the way the services interact between Birmingham and Leamington does lead to quite a few odd situations further along the line to Reading.
What seems to happen with somewhat noticeable regularity is that I will end up at Birmingham waiting for a southbound service to find the Manchester-Southampton scheduled to depart a few minutes ahead of the delayed Newcastle-Reading. The temptation is to go for the first departure but I have learnt the better thing to do is to wait for the Newcastle-Reading, if the service goes within 5-10 minutes of the Manchester-Southampton then at Leamington Spa it goes through first meanwhile the Newcastle-Reading now carrying most of the passengers is held North of the junction.
Even when this doesnt happen the Newcastle to Reading services tend to be quieter in terms of passenger numbers probably connected at least in part to the calling pattern, missing Leeds, Chesterfield, Burton on Trent, Tamworth, Birmingham International and Coventry. It certainly means I will try to use them for my somewhat regular journey from Reading to Birmingham New Street and back as it is normally much easier to get a seat on these services rather than those calling at Coventry and Birmingham International.
I guess if the original plan to serve Birmingham Airport via bus link from a station call at Solihull had come to fruition this would have made a difference but that seems a long distant memory now.
In this case it sounds like the delay on the Southampton/Bournemouth service had also resulted in it being held at Leamington waiting for the on time Reading service to go ahead, I guess there wouldnt be time to make a judgement to keep the normal order of services (probably at the cost of a 3 minute or so delay to the Reading service?).
One side effect from all this confusion leaving Birmingham seems to upset the departure boards and on one memorable occasion at Banbury to complete confusion as to which service might make it first.
Once the XCs have fought their way to Aynho Junction competing with each other and all the Chilterns they then get a reasonably clear run given the few and far between FGW stopping services between Oxford and Banbury.
At which point a service coming off the Cotswold line or ECS from the Oxford carriage sidings rolls the dice once more.
I can therefore see why by Oxford desperation would be setting in as the following service gets later and later waiting for the preceding service to clear Leamington, Banbury and finally Oxford meaning that the decision is taken to depart as soon as possible.
Thinking more generally about the XC Reading-Birmingham NS route I notice I end up staring out of the window at the Northern edges of Oxford waiting for a platform from time to time so improvements wise:
Long term Oxford could do with more platforms, perhaps akin to those reopened centre platforms at Taunton allowing services to turn back on what are now the through lines, and in situations like this for the services to get back into the intended order.
It tends to be quite overcrowded Reading/Oxford to Birmingham and vice versa at peak times. Part of the problem is that while there seems to be plenty of capacity Banbury to Birmingham using the Chiltern Railways services few have connections from Oxford, if the Oxford Banbury stopping service was expanded this would perhaps help to increase the loadings on the Northern end of the Chiltern services and alleviate the overcrowding on the Cross Country station.
Of passing interest just this weekend Chiltern have been taking their services South from Banbury to Oxford, adding to the Oxford.