Voles perhaps?

I googled (did you google?) and the first posting to 'diseases of prunus mume' indicated all prunus and most frequently 'mume' are subject to winter and spring fungus attack. Particularly mume which blooms earliest and susceptible to freeze thaw damage that cracks the twigs and allows entrance of fungus spores.

Do a bit of research and see if this doesn't describe what happened to your plant.

Call or e-mail the Parkses at Camellia Forest Nursery in Chapel Hill. They specialize in _Prunus mume_ and may be able to answer your question. The phone number and e-mail address will be on the Camellia Forest Website.