Why Do Nearly Naked Men Crucify People Upside-Down?

I'm struggling to make sense of the world right now...  Walking along York Place I see a group of men walking along in the bus lane, nearly naked, carrying a large and heavy wooden structure.  Catching up with them as they turn left to get to Multrees Walk I can their "friend" on an inverted crucifix with his blonde hair flopping down.  Lifting over the bollards wasn't easy and it clear takes a lot of effort.  Can anyone tell me why they would be doing this?  It seems too intense for a stag party, plus everyone seemed relatively fit.  Perhaps an end-of-season team event due to a lost bet..?