This passage is from Macmillan English Grammar in Context by Michael Vince.
"Every year thousands of children go to the doctor because of back pain and in fact this kind if problem is rapidly becoming one of the most common childhood complaints. And what exactly is causing (or causes) this outbreak of back strain and muscle fatigue? Most people are simply not realising (or simply don't realise) what is happening to their children.
I think both "cause" and "is causing", "don't realise", and "are not realising" are correct since the book says the progressive is used to express a trend.