Most likely (IMO) is mates mucking around, iv'e seen it before and it makes me angry.  We all moan about H&S but when you see the stupidity of some people, it makes my blood boil.
I'm  (or was until i find another job) a maintenance engineer and have a healthy respect for dangerous equipment.  But give a 'helper' such a piece of equipment and common sense goes out the window for them.  Air lines being blown at people, a quick blast from a pressure washer to wet a passer by...they'll both do extreme damage if you're too close.  Bollock them....and as soon as you're back's turned 

 .FFS they're at it again.  Brainless, that's all it is.