I was the one who messaged Cheese during the mission about this. This is what happened from my perspective:
I JIP'ed into the mission as Mortar Gunner, as it was the only slot that was left. After reading up on the Briefing I quickly went over to the rest of the mortar team. I dropped all my ammo etc. There was a general atmosphere of boredom in the mortar team. It seems the players weren't really aware that that is normal when playing an indirect fire role.
After 5 min or so of not doing anything, I believe it was Whitehead(I'm not 100% on this), said something along the lines of "well since we aren't doing firemissions let's make our own entertainment. Let's launch a mortar up someones arse." which was met with enthusiasm. I'm pretty sure I remember us beeing 10m or so away from the mortar when that conversation took place. I quickly put some 30m between me and the mortar when it was clear to me that they would be doing it. At that point I was already looking whether there were any Admins on. CHs also went away from the mortar while the other two(Whitehead, Bumbake) took their positions. Because CHs was only around 10m away when the mortar was launched by Whitehead up Bumbakes arse all three of them died, the mortar destroyed while I was wounded lightly.
Bumbake disonnected pretty much directly after the explosion took place.
To me Bumbakes version of events seems extremely unlikely. There was a conversation about the action beforehand, which took place some distance away from the mortar, so that everyone had to move in order to get into position. I had enough time to move away some 30m before they were in position to launch the round.
Launching a mortar up someones arse is not an accident. You have to position the player exactly.
This is how I saw it and reported it to Cheese.