Can't explain it but suggest you try trahing the preferences before the next burn.
It's dead easy and won't take a minute.
Many weird things happen as a result of corrupt preferences which can create a vast range of different symptoms, so whenever FCP X stops working properly in any way, trashing the preferences should be the first thing you do using this free app.
http://www.digitalrebellion.com/prefman/
Shut down FCP X, open PreferenceManager and in the window that appears:-
1. Ensure that only FCP X is selected.
2. Click Trash
The job is done instantly and you can re-open FCP X.
There is absolutely no danger in trashing preferences and you can do it as often as you like.
The preferences are kept separately from FCP X and if there aren't any when FCP X opens it automatically creates new ones . . . instantly.