remove items dependencies
All elements not on the main storyline must be attached to something on the main storyline. That is an essential design from FCPX. 
You can move elements not on the main storyline to any position and it will link to the underneath main storyline element. 

However the link point can be shifted to an extend. 
Select the attached element you want the link to sift. Hold down Command-Option and click somewhere in the attached element. The link to the main storyline will change to the position you just selected.