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Can anyone help me understand what happened during a re-maiden flight of a quad rebuild today.  Specifically I experienced strange behaviour of the APM 2.5 after it had landed the quad duringan RTL.

Prior to attempting the RTL I had previously gone through each of the other modes in order - Stabilise, Alt Hold, Loiter. Each worked perfectly. In Loiter the quad was nailed to the sky as if it were mounted on a post, even with a gusting breeze it held position perfectly.   In the RTL flight itself the quad oriented itself perfectly and held altitude at 15 meters.

My experience is as follows, I would appreciate any thoughts or suggestions as to what may have gone wrong. 

Switching to RTL the quad climbed to 15 meters and held position momentarily. It then rotated to face the home position (me) and made its way back. Overshooting slightly it corrected and hovered above and just ahead from where I was standing, I would say pretty much bang on launch point.  It then descended slowly and under apparent full control until it landed gracefully, running the motors down to an idle speed. 

Not one to be outdone, my disaster genie struck at this point.  I waited for the quad to disarm, but it did not.  So I attempted switching out of RTL, forgetting that the mode switch was still set to Loiter (from which I switched it to RTL).  The motors immediately increased in speed, vibrating the quad like an earthquake, but not lifting off. I tried to disarm with the Throttle/Yaw stick but it did not do so. The vibrations were getting worse and then the quad promptly flipped over 180 degrees and proceeded to strip away its carbon reinforced propeller blades one by one! Curses!  

Eventually I [thought that I] realised what I'd done - flipped the mode switch to Stabilise and then using the Throttle/Yaw stick I was able to disarm the APM.

I have carefully read through the Arducopter wiki and cannot find anything that may explain the post-landing behaviour.  http://copter.ardupilot.com/wiki/rtl-mode/