Timber is a natural product, and it is normal for it to be subject to warping, twisting and shrinkage, particularly in the summer months when the weather conditions are dry and warm. It will normally swell again in the cooler seasons when it gets wetter.
Splitting can also occur due to the natural position of a knot in the wood, and this can happen at any time.
The best way to try to minimise the impact of nature on your fence is to keep it protected and treated with good quality wood preserver, treatment or paint.
There are many excellent products available on the market. Although we do not stock these ourselves, you can source them from any reputable paint shop or garden centre.