Copter keeps on yawing for Hexa-X frame in SITL

Hi,
When I run a mission with arducopter Hexa-X frame in auto mode, I find that it follows it exactly. But in the entire mission it keeps on rotating about z-axis (yawing). I am using mission planner for running the SITL.

Can anyone please help me understand why is this happening?

Thanks,
Sayan

This happens for octaX frame too.