Why is the A400M APU on the side of the fuselage instead of the back?
The A400M is designed to operate from unpaved surfaces.

With the RAT on top, this avoids FOD issues, and provides ground clearance.

For the APU, it's space and proximity. You've got the wing box above the fuselage, plenty of space to add things there. Also easier maintenance access. Proximity as in less wiring and tubing to run inside the airframe, saves on weight and complexity. And again FOD. The inlet would be away from the debris and dust the wheels kick up.



Source: gizmodo.com