Why the aircraft starts landing after calling return home function?

The return home function's behavior is decided by firmware, MSDK cannot decide. Every aircraft's return home function behavior is different, you need to check the User Manual.

A normal logic is:

• If the horizontal distance between aircraft and home point is greater than 20m, the aircraft's altitude is lower than return home altitude, the aircraft will climb to the home altitude and return.   
• If the horizontal distance between aircraft and home point is greater than 20m, the aircraft's altitude is higher than return home altitude, the aircraft will return home with the current altitude.
• If the horizontal distance between aircraft and home point is smaller than 20m, the aircraft will start landing.