The XKF3 sensor fusion algorithm not only computes orientation, but also keeps track of variables such as sensor biases or properties of the local magnetic field. For this reason, the orientation output may need some time to stabilize once the MTi is put into measurement mode.
Time to obtain optimal stable output depends on a number of factors. An important factor determining
stabilizing time is determined by the time to correct for small errors on the bias of the rate gyroscopes. The bias of the rate gyroscope may slowly change due to different effect such as temperature change or exposure to impact.
For more information regarding sensor bias also see the following article:
Understanding Sensor Bias (offset)