rockm

"after the foliage has gone dormant but while the soil is still warm enough to promote root development"

You can't think in terms of days or weeks for warm temperatures. Winter is coming, along with deep freezes. Newly sprouted roots are very susceptible to die back especially in the winter, since they will not have four to six weeks of warmer temperatures. If you root prune now, you will have to find a frost-free environment to overwinter the tree.

"Why would one wait until bud break to prune the roots? "

Waiting until bud break ensures the tree has already distributed the nutrient stored in the roots the previous autumn. In spring, the plant has prepared itself for a growth push--removing the roots in the fall could slow it down in the spring, since you are removing stored energy. Buds swell in the spring using that stored energy. When they start swelling, they are drawing from reserves. Pruning before buds swell leaves all those reserves in the roots. That's why we usually wait until just BEFORE buds break before root pruning--to take advantage of the redistributed nutrients. Top growth will push root growth...