Why throttle rises automatically

Hello - I’m using pixhawk with ArduCopter ver3.2 and a quadrocopter.
As an initial throttle test I did the following:

  1. attached the quadrocopter to the top of a table
    so that it would NOT take off.

  2. increased the throttle stick on my RC transmitter up just a little bit and left the stick
    in that position.

  3. listened to the sound of the four motors turning.

  4. The four motors started fine but gradually started to turn very fast (even when the throttle
    position was left unchanged). So fast infact
    (listening to the sound of the motors) that I had to turn down the throttle stick.

It appears to me that when the quadrocopter is not allowed to take off freely, some algorithm
is forcing the motors to surge.

Has anyone else experienced this problem ?

The weird thing is that when I use ArduCopter ver3.1.5 with the same set-up, the surging of the
motors does not occur.

Any feedback would be greatly appreciated.

What could have happened here is you had it in altitude hold mode and the barometer started to drift negative and the blades started to turn causing the motors to increase to try and correct for the height it was at. Hard to tell without logs.