In OS X Lion Apple introduced the concept of document versions, the ability to go back in time on documents and view or restore a version from the past. With this they introduced a “Lock” on documents that hadn’t been changed in over a week or so and requires you to unlock the document or duplicate it and save it elsewhere.
I personally find this lock annoying. It is possible to turn off, but perhaps a little bit too hidden. To change the duration or disable this feature completely, you need to open System Preferences and go to the Time Machine section, pretty strange considering even if you don’t use Time Machine you still need to come here to change this setting around.
Click the Options… button and uncheck the “Lock documents” checkbox or extend the lock duration.