Lewis wrote his in the 1930s.  I think his came from his own experience as he journeyed toward Christian belief. At one point he believed in God, but not yet the gospel. 

His enjoyment of Northern myths would also have contributed to his sense of a great and strong force that could harm humans, whether or not it was good. 

There's also the idea that many have reported that having a demon rest on you is a crushing force, whether it bows you over (George MacDonald, an influencer to both Tolkien and Lewis) or suffocating force (southeast Asian mythos, North American oral tradition). I wouldn't be surprised if both share an external experience or influencer that caused them to use that illustration in their own way.

Well, I think I might lean toward the coincidence option here. After all, whether you're talking about Oyarsa or Cthulu, both writers are talking about gods. The idea of a crushing divine force is certainly nothing new, and has been around at least as Christianity itself (which, I think it's safe to say, was the main influence on Lewis, even though he might not have been a Christian at the time).