My theory : it's the Coriolis effect, as in : the-earth-is-rotating-under-the-helicopter-while-it-stays-fixed-in-space. The clockwise rotation as seen from above would be consistent with the Inertial Circles observed in wind patterns in the northern hemisphere. Considering that the Coriolis effect has to be taken into account in modern long range ballistics like artillery shells or sniper bullets with flight times as short as 5-10 seconds, I wouldn't be surprised if it played a role in a situation like this.

Whatever the reason, half a minute is a long hover, I never stayed in place long enough to experience what you describe, and I learnt to hover with a Blade CP. It wouldn't hold it's heading for more that 10 seconds, let alone 30. I wouldn't worry about it.