As to what really happened can be divided into 2 major scenario.
Scenario 1 = All 3 horsing around then all 3 mistakes.
Scenario 2 (A)= No horsing around meaning only commander fault or whose error?
No checking of clear for men then press button to lowered barrel while Aloysius might still be standing.
Body and or uniform got click up by rising breech.
Scenario 2 (B)= As u know being in reservist might not be in full mode.
Aloysius might be in an unclear position and button pressed.
Scenario 2 (C) = Might want to test breech can lift own weight jump up on breech with intention to come down after breech rise but unfortunately unable to do so due to lack of time and or uniform stuck on breech.
could be any one of these scenerios.
no one mention inside Vehicle got cctv and or voice blackBox ???