I suspect that improbable readings from the ultrasonic sensors can also trigger this. I experienced this once when I powered up the Typhoon while sitting on a drivewau where the surface started sloping upwards at a steep angle just in front of the Typhoon.
On power-up the drone is likely doing a self-check or diagnostic, and if the sensor reports a object in close proximity, that could indicate a malfunctioning sensor. Tall grass might also trigger this.