There is a common misconception that CPU bottlenecking only happens at 1080p. This idea has been widely propagated because this is where it is most likely to happen, but that is only because 1080p has higher frame rates than resolutions like 1440p and 4k do; running at 1440p/60 fps is just as hard on your CPU as 1080p/60 fps is. Your CPU doesn't care about resolution, that's your GPU's job, it cares about frame rates. The higher your frame rates, the harder your CPU has to work, increasing your chance of a CPU bottleneck.
When you play a game, your CPU requests data from your hard drive/RAM and renders certain parts of a frame. It then sends the pre-rendered frame to the GPU so that the GPU can do the portion of the frame rendering...