The suspicion is that a recent large earthquake accelerated a slow process:
A month after the earthquake, there were almost 100 sinkholes spread over a 10 sq km (3.8 sq miles) area, with new ones opening every week.
The hole in Borojević's garden is now the largest in the area. When it first appeared it was 10m (33ft) wide, but started to grow almost immediately.
"My wife was in the house the whole morning, looking occasionally through the window," says Borojević. "Around 2pm she noticed something strange in the garden. We went outside and there was this huge hole in our orchard." Over the following three months, the hole tripled in size.
But the Borojevićs were lucky. Other sinkholes in the area opened up just a few metres from the doorsteps of people's homes and one appeared beneath a house