Easy to work out why they hit you on the head when the upper limb breaks!
Imagine a clean break mid limb on the upper limb...
The two ends of the break will move rapidly away from the archer. ...
BUT... The top of the upper limb is held in it's original position by the string, and will tend to stay where it was due to inertia (as it was stationary at full draw)*.
The broken end swings up and over the top pivoting about the nock...
And...
Thwack on the head
.
Del
*Adding to this effect, the string tension will be pulling the upper nock down, which will help to impart more spin to that broken limb tip, as it pivots about it's centre of mass.