This is most likely caused by the fact that you’re not using a pattern/pin/password lock.
The Timeout feature in JuiceDefender is not meant to be used with the regular, insecure lock screen (the “swipe to unlock” thing) – this is also stated in the warning text for the Timeout section. So the solution is simply to disable Timeout (as it can’t be of much benefit anyway in this case).

Then again you should seriously consider using some form of lock screen security – just think of how much personal information would be immediately available (all your email!) if your phone is lost or stolen.