If it has been in the same pot for longer than 2 years then gently remove it from the pot and check the root ball. The problem is probably stemming from the roots (pardon the pun) Drainage might be a problem Over fertilising is another thought
Probably either needle blight or root rot. With dead tips + green interior fitting the first condition in particular.
Hi, I had yews in pots many years ago and the same thing happened to mine. The problem was not looking after the soil / compost. I lifted them out of their pots and the compost had disappeared and the root ball was tight. I purchased three new larger pots added soil and compost, teased the roots all round the rootball and repotted them. I did this every two years for the next 20 and all picked up and still look healthy today. My children now have two of them in their gardens and I still have one all three are doing well. I have posted mine to show you.