Question
why would my well established dedora tree start dying? Is there anything I can do to stop it ?

Answer
Dear Monica, The only real things that would cause this would be a change in the soil pH, not enough light (rare in an established tree), a change in the amount of water the tree was getting, or some new insect or disease in the area  which is affecting your tree.  This cedar needs soil that is neutral to alkaline, which would be 7.0 or greater if you can do a pH test.  The biggest likelihood is that the area is now getting too much water.  All cedars need to be on the dry side to be happy, so if you have had lots of rain recently, that could be a big factor.  The only other option would be some pests or disease.  The main pest would probably be something like bagworms, but you don't mention those.  I am guessing this is a water issue and it will probably improve when it dries out.  Be sure to look at the plant carefully for signs of bugs, tap a small branch over a sheet of white paper and if you see tiny critters moving, then you could have spider mites and you should spray the entire tree several times to get rid of them.  (You can use Ferti-Lome Triple Action Plus because it takes care of insects, mites, and fungus if you suspect spider mites or fungus.)  That is about all I can think of.  Let me know if you have additional information or need more help.  Good luck, Melissa