I surmise in the present situation with IS making threats, someone in a crowd over there could simply drop something as they walked and any sharp noise be wrongly interpreted as a terror attack starting.
IS isn't going to bomb mecca....
and these mass deaths have been going on at the Hajj for a while now - the issue is the lack of crowd control by the Saudi authorities despite repeated incidents... they impose token measures then carry on as usual
	
 
 or maybe religion just doesn't work the way they think it does?