Must just be the combination of that increase as well as other words decreasing in usage I suppose. E.g. perhaps we're a bit less keen, but also much less passionate, so keen ends up making the cut.
Saying "I'm keen to do x" or "he's keen to do y" is a fairly common normal usage but having said that, wouldn't normally be my first choice of language as a millennial, maybe does sound a bit old fashioned
Oughtn't really affect emotive language like 'keen', though.