I think it still could have been winter damage, but mostly I think it is probably a soil-borne pathogenic organism like a water mold , bacterium or fungus. These are common. Do you have damp, heavy soil or a location down in a hollow where the air is often cool or cold and damp?

Thank you Bboy.

I do not know about the soil. It definitely does not seem like heavy clay. But the plant is surrounded by grass. I had also put some leaves near the root base for some winter protection. 2 of the branches are still green with some new growth. Is there anything I can do for them?

If there isn't a big clear area around it the grass may be affecting it.