It doesn't have to be breeding. It may or may not be. Rival females will do it to eachother as will rival males. In this case the dominant fish has just been faced with a new tankmate. In the fish's eyes, a challenger for his/her territory.
Many mbuna of both sexes may have eggspots. Breeding shaking from males involve a male mbuna shimmying in front of the female and then back to his pit, then back to the female, then back to the pit, repeatedly so that she is enticed in and submits to spawning. When the female does not comply a male will give chase, seemingly out of frustration. While the male is "luring" he is the only one shimmying. They will both shake while in the act if spawning but the breeding jumps are much shorter than those of display.
When challenging or arguing fish will shimmy and usually both fish will do it. They may also circle, neither wanting to back down.
It'll take a bit of observation but you'll soon learn how to interperate your fish's body language. Some folks like to think that the only appeal to these fish is their color but they have an entire language that is fascinating and fun to read.