Could be a lot of things-
Have you tested the soil pH? It is quite close to that concrete. Look in the white pages by county for your cooperative extension agent and get your free soil test done- this is vital when learning about a new property.

Any chance the upper portion was pruned last year?
Something dropped on top of it?
Does it receive equal light on all sections?
Water being blocked on one side maybe?
Other plants competing?

It's not unusual or alarming for a portion of a neglected and/or poorly sited plant to die IMO.
I would prune it after it has finished flowering and then, after correcting any pH problems, watch it. If your weather is not too hot by then maybe move it at the same time, but be gentle, set the rootball on fabric or something solid to pick it up, and don't replant it too deeply.
If it is too hot at that time wait until fall to move it.

But first get that soil test and find out what you are dealing with.