this has been mentioned before, but i'm having trouble with the search, so...
i think it's something like this
the fence is the boundary of the course - you were probably near the fence, still in bounds but the fence interfered with the swing
in real life, i suspect the boundary fence is declared an integral part of the course,
but... wgt as the tournament committee have probably declared the fence as an obstuction
and given you free relieve. it also appear they have nominated a drop zone as the tee box.
all a bit bizarre, like a few other "implementations" of rules
note also in real life the road across the 1st and 18th is an integral part of the course, so no free drop allowed. wgt have created their own local rule (this is allowed within the rules of golf) and declared the road an obstruction, giving a free drop on the fairway behind the road.