Weather vane.
These cranes aren't always unlocked(left to slew,weather vane).Even fixed jib(not rising/lowering,luffing)cranes are always left unlocked.It is down to the operator to decide.Mainly dependent on the weather and surroundings.This crane would slew over a public footpath.Not ideal unless in bad wind conditions.Any damage to the trolley,top of the crane.Would cause a catastrophic failure of the jib.As it is supported by the counterweights through the cable connected to the trolley.The jib alone weighs over 4 tons.And if you know about fall factors,you'll know the result.Had it been hit whilst locked and positioned over the site,it would not have landed on the highway.