Hickory, of all the bow woods I know, loves to be dry but 5% or 6% is where it likes to be. At 2%, if your meter is good, would probably break any bow.
Being a board I wonder how the wood was cared for off the stump. If the trunk laid on the ground for a week or more in the summer and wasn't stored properly after that could be the cause. Healthy hickory is very difficult to break.
Will you post pics of the break and where it is on the bow?