Pretty sure there's only two (major) factors:
1. The proximity of the closest city(s) to yours. Anything btw 4-9 tiles from your city-center will exert loyalty pressure.
2. The amount of that pressure is also highly based on population [of the nearest city(s).]
[There's some unique conditions too - like Eleanor(France) can exert city pressure from her great works... maybe all Civs can... can't recall... but the two major items are above, I believe]
I actually recently watched some good videos on YouTube on dealing with that.
Sometimes, on higher levels - you NEED to settle a city on new continent and maybe everything is -10 to -20 range. (high)
Best way to deal with that pressure is to get your own city population up... FAST. Chopping bonus food resources (wheat, deer, fish, etc) for instant food is good method.
You can also move a governor in (takes 3-5 turns), or slot in a policy card to help (there's two), or move a food-heavy trade route from that city.
If you have gold, BUYING monument and granary is immediately helpful.
Doing none of these things... is going to possibly cost you losing your new city though...
Good video:
[I've never seen a cap on loyalty of -10... maybe it's at a lower-level.]