I have a Bosch 1200 AE which is practically the same router, if you undo the screw that holds the locking mechanism lever in place you will find a brass hex headed screw/plug, you can turn the screw/plug clockwise a little and replace the lever in a "tighter" setting so the lever locks harder, maybe the locking mechanism has become worn a little and will no longer hold.
Look at the bottom left with the number 801 above it to see how it works:
Look at the bottom left with the number 801 above it to see how it works:
