When you grow the right edge of a phase to the right, it will move all contents in the next phase (if any) to the right, giving you more room in the current phase to add shapes. However, if you shrink the phase by moving the right edge of the phase to the left, you can end up overlapping content that is in the phase. In other words, it will not simply move the phase line boundary, but will shrink the phase, pulling objects to the right of the phase over to the left as well.
In order to make the phase 'smaller', simply delete the phase (select the phase name area and use the Delete key), and then re-add the phase where you want it at the point to the left of (smaller than) it was. Now the phase line will be at the correct point.