Hi,
The first thing I would try is to open iMovie while holding down the Option and Command keys, and selecting to delete preferences in the box appears. That sometimes cures glitches.
You didn't mention what trailer theme you were using, so I can't test to see whether it occurs on my computer.
It might be a glitch in that one theme or in that one project. You could try changing themes, or using the same theme in a newly created project, or a different theme in a newly created project.
Try duplicating the project and converting it to a movie and see if the issue appears in the movie version, or whether you can delete it there.
-- Rich