I can't answer the question, but I can suggest some questions that may help you find the answer for yourself with further investigation.
- What are the common features of these cases? Careful investigation could provide clues.
- What do you mean by "many"?
- Are these situations widespread or regional?
- If regional, what has changed recently, e.g. in management of the cows and their calves? - unusual weather extremes? - inbreeding, with a defective gene being passed on? - different feed, whether from grazing or hand-feeding? unusual stress, overcrowding?
Thank you very much for providing clues for further investigation. I will update best information as much as I can once I gather from various farms/fields located in across the country. Really helpful suggestions.