Muscles cramp if they are asked to do something they are not used to - either in terms of volume, or in terms of the nature of the activity - or a combination of both. If you had trained to run on "a gently falling field of very horrible tussock" then you probably would not have developed cramp. Once a muscle has cramped it will tend to continue to do so for quite some time, even with trivial movements or even at rest. Despite what some say there seems to be little evidence that it has anything to do with salt or potassium depletion.