Its not a mental question, but the answer involves things that are fairly well known already.
The media is the fourth estate, this alone is almost enough to answer the question.
If you are in a position of power and responsibility, or are privy to very interesting information, then there are many opportunities for the weight of responsibility to produce internal conflicts between opposing goals, beliefs and forces. This can result in decisions that speak volumes about how things really are, what really counts, what forces will run roughshod over nobler aims. Usually the journey to the position of power will have provided plenty of opportunity to ensure that the person in question will 'do the right thing' and in particular a sense of loyalty to the nation on some level can be stretched a long way. Going to Oxford or Cambridge will also tend to increase confidence that the humanoid in question will be a safe pair of hands.
On a more general human level, not exclusive to those with some power and influence, knowing a secret or any information which could be of benefit to another can really heat up our brains. Whilst there are all manner of occasions on which people manage to keep their cards close to their chest, sometimes we end up with a burning desire to share information with others, including those affected, even if the consequences carry more personal risk than potential benefit.
Of course none of this in any way rules out other motivations that are specific to the situation & people involved, theres plenty of room for you to decide that he did it to help the Tories or the Lib Dems or whatever. I don't think there is anything wrong with dwelling on this stuff now, but at the same time I can see why people would be keen to focus on one target at a time, not fight on too many fronts at once in case the overall effort is undermined. This can be easier said than done, for some of us may question whether anything will really be solved if we ignore all the double-standards and hypocrisy that shows up along the way. We might be rather interested in the full reality of how things are, even if shining a torch into certain dark corners at the wrong moment causes the struggle to become fragmented.