So, for example, pick a subject, let's say a modern language. The old fashioned exams were not so systematic. You answered questions as well as you could using the language, and the examiner marked you impressionistically on your overall grasp of the language. In the modern system there are stock phrases and sentence forms you are marked on, and students are encouraged to learn these as boilerplate (that's how my kids were taught, and they did well in the exams). This type of exam is cheaper to mark, and easier to teach to. It is also financially lucrative for the examiners, who can create text books which concentrate not on mastering the subject but satisfying the marking system.
This is why increasing proportions of pupils get high grades. It's not dumbing down as such: it's improved technique, combined with a system which increasingly rewards technique. This is common where there is any system of grading or evaluation - if you set people hoops, they will get good at jumping through hoops. And as I say, the exam boards have a triple financial motive: the new type of exam is cheaper to administer, it's more attractive to the purchaser (schools, because they can get more grade As), and it provides a second source of income to the examiners (who are generally freelance) in creating exam guides and text books which schools must buy to learn the marking triggers.
What it doesn't do, in my opinion, is improve the quality of learning. I personally think this is quite a big problem, but when journalists call it dumbing down it makes people defensive.