Apple IMovie

I don't know what is causing it but for a start you could delete the preferences to see if that helps.

When  iMovie starts behaving unexpectedly in any way for no apparent reason, the problem could be caused by corrupt preference files. 

The cure for this is to delete the preferences. 

 Hold down the  Cmd and Option keys as you launch  iMovie and this window will appear.

Click “Delete Preferences” and  iMovie will open in the default (as-new) mode.

You may have to navigate to any project you were working on and reopen it.  Your project libraries by default are stored in your Movies folder unless you have specified somewhere different.

If Ian's good suggestion doesn't cure it, it could mean that there is something corrupt at the beginning of your movie. Try trimming off a few frames at the beginning and then sharing out. See if that cures it.

Also, there could be some general corruption in the project itself. Try putting your media into a newly created project and sharing out.

-- Rich