befall vs overburden

befall

verb
  • To fall upon; fall all over; overtake 

  • To happen. 

  • To happen to. 

noun
  • Case; instance; circumstance; event; incident; accident. 

overburden

verb
  • To overload or overtax. 

noun
  • A sterile stratum that lies above the stratum being investigated 

  • The rock and subsoil that lies above a mineral deposit such as a coal seam. 

How often have the words befall and overburden occurred in a corpus of books? (source: Google Ngram Viewer )