@Liquidxlax Mixture of all of those. Traps, rhomboids, lats etc all get affected by them. To over simplify, tig aul bitties are constantly fighting against those muscles to pull you into a hunch with rolled shoulders rather than a proper erect posture with shoulders back.
Oddly enough a lot of more well endowed women have terrible back muscles incapable of supporting the offending articles. Source: having to perform regular physio massages on women with this problem.
Also never underestimate the importance of a strong back when lifting regardless of form especially in a situation where the weight has to be help outside your centre of gravity, your back WILL be use during the lift. If your only reference for lifting is a barbell/rack squat you'll not have much fun if you try something like a keg carry.