Or, yields have dropped in the drought so farmers are forced to increase acreage? Probably that?
That was not the claim. The claim was that an decrease in supply of one input (water) could drive market conditions which lead to an increase in consumption (in the course of production) of another input (land use).
I don't think it's that much of a stretch to think that instability in market prices for a good may cause odd market behavior that has consequences in related areas.