I could be a learnt behaviour for a particular populations of foxes. They may be learning that cats = easy-ish food.
Or that they consider cats to be competition for food as cats will catch a lot of their pray and are defending their territory, most cats probably escape, and it just the weaker/slower/old ones that are getting nabbed.
Considering that the fox population is 250,000 - 750,000 depending on the time of year, about 1 in 7 live in urban areas. Cats are numbering 11 million. So an urban fox will probably encounter a lot of cats.
The number of cats being involved is a few 100, is a tiny number considering the population.
And when something becomes a news story people will link their incident to it. These deaths could have been happening for decades, but no social media/internet/news to link them all.