I've now found out what the most common cause for this is, pneumothorax (collection of air in the pleural cavity). However there are different types of pneumothorax and each one causes a shift in a different direction. Here's why...
Usually occurs as a result of trauma (e.g. a stab wound) and results in an accumulation of air under pressure in the pleural cavity. The wound (or opening) acts as a 'one way valve' allowing air in to the thorax but not out.
In this case the mediastinum would shift AWAY from that area (e.g. if it was a left lung tension pneumothorax the mediastinum would shift to the right). This is due to pressure gradients and the fact that it's encourage to move from an area of high pressure to low pressure.
PSP (primary) refers to those without chronic lung conditions
Secondary refers to those with underlying lung disease
In this case the lung would collapse but there's no 'one way valve' effect letting air in and not out. Therefore the pressure on the side of the collapse is lower than the contralateral side...meaning that the mediastinum shifts TOWARDS the collapsed lung!