When you add water to cement in a sand cement screed it undergoes two basic mechanisms which cause it to shrink. The first is that water evaporates and the second is the apid hydration of the cement. These cause the screed to shrink in its early life. This effect is called primary shrinkage. Coupled with lack of compaction which leads to intrinsic weakness in the screed these are the primary causes of early age cracking in cement based materials.
Once the screed has set and become rigid and has ostensibly dried there will always be some moisture present and as long as this is there the hydration reaction continues. This hydration coupled with carbonation of the screed means tha for very long periods (years and years in some instances) the screed continues to shrink very slowly. This causes further cracking over time. This is called secondary shrinkage.
The ways to minimise this sort of cracking are to ensure that the screed is cured properly initially, ensure that it is fully compacted and does not dry too rapidly. This way it can build up enough internal strength to overcome the effects of the shrinkage. Lastly it must be remembered that this secondary shrinkage will always occur regardless of you efforts so correct screed design ais also essential I.e. joints in the right places and bay sizes not over sized.
Polypropylene fibres can also help but these deal mostly with primary shrinkage.
Hope this helps although much oversimplified.......
