This has a whiff of the perennial 'kids today' rant. I know Google NGrams is hardly a robust research tool, but a few phrases that are fun to speculate about ...
I think this shift in word use is natural. Words become popular, and then get overused to the point that using them calls to mind a lot of connotation. To avoid that, speakers looks for a comparable word and start using that instead.
Then, eventually most good words (small enough to be known by the average person) get used up and speakers end up creating new words for the same reason.
Witness 'comment / commenter / commenting', which in the sports world has completely given way to 'commentate / commentator / commentating' to the point that those 'new' words past Google Chrome's built-in spell-checker.
'Failing student' - big in the 30's, went out of fashion? http://books.google.com/ngrams/graph?content=failing+student...
'Unprepared student' - relatively constant over time http://books.google.com/ngrams/graph?content=unprepared+stud...
'Underprepared student' - what happend in the 70's to get people writing this way? http://books.google.com/ngrams/graph?content=underprepared+s...