Apparently it was caused by malnutrition: https://www.taipeitimes.com/News/taiwan/archives/2020/12/24/2003749304.

"Malnutrition is likely to blame for a recent spate of water buffalo deaths on Yangmingshan (陽明山), the Taipei Animal Protection Office said on Monday.
...
The animals developed nutritional deficiencies after eating low-quality food over a long period, the office said.
...
Chinese Nationalist Party (KMT) Taipei City Councilor Keng Wei (耿葳) said that park officials have since last year been erecting fences to restrict the buffaloes’ grazing area, yet are blaming the mass die-off on the weather.

Keng accused the park of deliberately hiding the fact that its misguided policy has starved the animals. "

It does seem odd that 25 animals would just die due to malnutrition, just like that. That's not something that happens overnight.