Cedar timbers often split and crack, in the industry it is referred to as “checking”. It is a result of the drying process of the “green” fresh cut lumber. It occurs more commonly in larger pieces; due to their size they dry unevenly. Starting with the outside and working towards the center.
A key thing to know is that wood that checks do not affect the structural stability of the wood. It is just the release of internal tension build up and the wood drying resulting in shrinking. That being said you can take a few measures to reduce the checking.
Keeping the timbers out of direct sunlight on hot day and keeping them from getting wet on rainy days will help reduce the checking since both result in rapid moisture content changes. Covering them with a tarp or keeping under cover will greatly assist in this effort. Staining the wood before it has been put into place, will also help keep it from checking as many stains reduce the ability for moisture to transfer in and out of the wood.