Don
By trying to tiller the bow too soon after tempering. Hot wood dries out fast and when you temper a bow the back gets hot = dry. The outer surface of the wood is where it is the driest and dry wood becomes brittle in tension hence a tension failure.
V true.
I usually try to protect the back if possible, supporting the bow on a narrow plank edge rather than a 3x2, allows heat to blow past the back, a few layers of tape can help too
Sometimes clamp thin slats to the sides of the bow, this helps direct heat along the belly, (which can speed up the process a tad) as well a keeping it off the back.
Having said all that, it is hard to stop some leakage of hot air (happens when ever I open my big mouth
).
I especially take care with Yew... don't want to disscolour that pretty sapwood
Del