I remove people usually when I see no point in having them in arm's reach. Usually for that, it's either that we never talk and have no interest to talk to each other again for about a year or two and then I drop them, but sometimes, it can be a "Yeah, no, goodbye" thing:

I removed an older relative once when he commented on a post that a really good friend of mine made about children in Syria having to live through literal hell in what should be the most innocent time of their life, also relating it back to the experience of her friend in turn whose family came from that region of the world and making the case it was important to help refugees, especially children. From this, this older relative of mine who happened to stumble across this since I interacted with it said in essential summary: "Wait until those 'innocent' kids turn into suicide bombers on our streets and see how fast your attitude changes about letting them in". I didn't see or hear about it until she called me up in tears over it, and in that call as we were talking, I removed him.