It happens everywhere including universities. It is definitely not better teaching or brighter students and it is easy to prove. One of my professors recycled and old exam form in physics1 course from 15 years ago - the result was a huge failures rate and an outcry for an "unfair" exam. The problem is that teachers get less and less backup if the grades do not meet some expected (ever rising) standard. In that case it is always the teacher's fault, either by poor teaching or by making exams "too difficult". The authorities even encourage this so they can boast improved statistics. It is said that if you tie a donkey to a tree inside a university campus and wait 4 years, it will come out an engineer.
I the case of universities, the entry requirements keep dropping in recent decades. The increased number of students does not maintain the mean student level - the good student has always came to the university, the "extra" students come from the bottom of the stack, so the average level decreases.