Do you know the strain of the plant, or whether its indica or sativa dominant? If its indica dominant, then Id say that shes using up the nutes in the leaves for that 'last big push'. All MJ plants do this in flowering. If its a sativa strain - which take longer to flower - then I would have to say that its some sort of nute deficiency. As most strains are a indica/sativa cross, its probably just doing its natural thing.