Bear in mind that your bare-root nursery purchase may have had roots cut in the process as Viron suggested. ...as "bare root" often have had dominant roots shortened, it is trade practice to spray an adequate amount of rain protection somewhere along the way before final retail. Should bare root have been sold without rain guard and non greenhouse grown by the customer (ie. outdoors west of the Cascades) all trees incur a state of "transplant stress" and not only the process of damaging roots by digging cause this, but critters such as root nematodes to voles on established trees are even responsible.

The stress in prunus are of most concern and in my opinion prunus should never be retailed this time of year. I have even bought a toka plum in a 15 gallon pot and had major die-back and there are is not much in the way of public statement on plums in the PNW as most documentation applies to sweet cherries which are more inclement than plum, but plums can be problematic too.

The die-back (prunus) is almost always certain to involve stress from the root first (ie pots etc), then bacterial, then fungals usually later. The aim is to "greenhouse" your purchase between the root stress  the bacterial infection. Simply sheltering your tree with plastic in a way to not increase body temperature is what I mean (to keep dry).

I wish there were simpler insights that I can reference you to but can't. The retail chains have kind of explained this one to me over the years of growing my own, and the sprays are usually accomplished at the wholesale growers, but just be careful. 

Now finally: Once sheltered at least it is now obvious there is no need to wait for dry weather, in which case "prune anytime". (smile)