It was climbing to RTL_ALT which you have set at 10m. What issue? You should update the firmware, 3.6.11 had a serious issue. 3.6.12 at a minumum but current Stable would be the best plan.
Arducopter 4.0.4 stable version got released, you might want to test that after upgrading to 3.6.12 and saving your parameters. That way you can come back to 3.6.12 in case you want to…