Really strange beahaviour from my Phantom2 after 10-15 mins. I was only 100-200 metres away from it when it started going crazy. It almost did a flip, drifted off to the left and started going down. Not evern the Zenkuse H3-3D could cope.I tried to enagage 'home' which didn't do anything. After a 2 mins of it doing all sorts of crazy manouevers I turned off the remote control forcing failsafe.  At this point it calmed down and fortunately started flying back towards me.
Weather was cloudy, but good, no wind.
I'm on the latest firmware versions (3.12). Everything is calibrated. The horizan is off, known problem with my gimbal.

Any explanations for this ?

https://vimeo.com/124098037

Whew, that was doing some extreme gyrations!  If you hadn't already said there was no wind I might have suspected high wind gusts.  I just looked at the PK-index for the last couple of days and everything was in the green so it likely wasn't solar activity.  And it looks like you were out in the country far away from any of the usual sources of interference like power lines, antenna, cell towers...

Any recent UFO activity?

Looks to me like flying in ATTI mode in high winds, you can see the wind turbine going like the clappers in the background, and most of the flight seems to be going backwards as the phantom is blown away.
Its not until you go into RTH that the phantom increases its angle of attack and makes some forward motion against the wind.
My tip would be to fly in low to zero winds from now on.