First, agricultural pathogens would not need to be genetically modified (the pathogens mentioned already exist in animals).
Second, they could perhaps be localized more easily (sheep and goats live in herds or on farms, and the viruses mentioned don't have long-range vectors in the same way that flu for example spreads via migratory birds, or humans on airplanes).
Third, economic warfare is an established strategic objective.
Nonetheless, I did not see any direct evidence that these pathogens were being used strategically, and there is another possibility - that handling them for defensive purposes is simply easier in countries that have looser regulatory considerations. For example it may be easier to test vaccines in a bunch of sheep in a field in kazakhstan than in Surrey, given that vaccine trial involve challenging vaccinated animals with the actual pathogen to see if the vaccine works, though it is not obvious why the military would be doing vaccine trials in sheep.