come upon vs hop on

come upon

verb
  • To befall; to affect; to happen to. 

  • To discover or find, especially by chance or accident. 

hop on

verb
  • Used other than figuratively or idiomatically: see hop, on. 

  • To begin playing a video game. 

  • To join a phone call or video call. 

How often have the words come upon and hop on occurred in a corpus of books? (source: Google Ngram Viewer )