My dragon tree bark is dropping. What is likely cause and how can I prevent. Is Yates anti rot best product

If this is caused by fungal related disease, Try 'Yates Anti Rot'. It is a systemic fungicide which is absorbed by the foliage and travels down to the roots, can be used on various plants to control phytophthora root rot, crown rot and collar rot. 

Click here for more info on this product:  https://www.yates.com.au/product/garden-disease-control/yates-anti-rot-phosacid-systemic-fungicide

However, if this is caused by pest or physical damage, best to locate the cause (ie, the type of pests) before better product the tree trunk.