Originally Posted by tour de low
The 5th bike shop I went to in two days explained it to me. You tighten that "dust cap" before you tighten the arm bolts. It says to do so in the instructions. It doesn't say why.
I saw that in the instructions as well. Tightening the left arm's pinch bolt on without first tightening the dust cap is analogous to tightening the pinch bolts on your threadless stem without first tightening the top cap... Makes sense that you saw what you did.