Despite the many theories on this, there is only one that has any evidence: the temperature swings. There is no official count on them either. 

Cows are not really made for a wide, fast, temperature swing. If it was the winter and say a swing from -10 to 30 to -10 over the course of a day, everyone would get some varying degree of illness, from their version of the sniffles to pneumonia. Heat is a different story, as like humans, there isn't really a great way to combat it, aside from building a giant air conditioned dome over them. Shade helps some, along with water mists and the color of their hides, but currently veterinary medicine and theories deal more with the air temperature, which is why you see fans on dairy barns as air temperature and moving air helps the most.

All that being said, packing plants are still backed up so food supply wise it means very little. Also, this isn't a freak event, it's just getting more publicly this year as...shortages are the "in" thing.