This time it's not traps - this is inside the Druids Grove, steps away from the forge and where Auntie Ethel first appears near the woman handing out gruel so I know it's not trapped.
Also Wyll was going about starting fights with people just by standing near them, it was all very strange.
I've had this happen in that exact spot. Only it didn't involve a guard. What I think is happening is that your party members are being trapped up there and unable to come down for some reason. Like the pathing for them got broken and eventually the game just makes them jump down. It especially seems to happen with hunter and warlock pets. Possibly because the pets weren't set up to use ladders.
It happened to me a couple times there where my party suddenly took damage after I headed to the forge down there, and I had no idea why. Then one time I turned around in time to see my giant spider land next to other party members, and they all took damage. The same thing happened once with my hunter in the Goblin Camp. My team was up in the rafters, and came down a ladder. The spider was stuck up above, then all of a sudden he dropped on top of the rest of the group and they all took damage.
Obviously there something off with the pathing for non-selected party members. But I think the damage might be intentional. I was meaning to check it out, but then forgot till just now. I'm thinking this may be a mechanic that goes along with pushing or knocking enemies off high places. I mean we all know that you can do some serious damage to an enemy by pushing them from a high place, but I'm thinking that the point of this is that you can also use that falling enemy as a projectile against other enemies. Only like a lot of other things, it can also hurt your own party members if they are in range of the falling creature. Just like all of our other attacks can hurt our party members if you accidentally shoot them or they are in your AOE.
I can test it by ungrouping then jumping on my own party members. Then try jumping on an enemy.
And if this is the mechanic, then I don't have a problem with it. But then they should fix the pathing so that pets that can't use ladders or climb don't deliberately jump on top of other party members.
In your case did Will have a pet out at the time? I'm assuming the guard got involved because the pet (or whoever) jumped close enough to him that he took damage.