Why is my bristle bush learning

Looks like a healthy cactus! perhaps mammilaria pilcayensis: that's something they may naturally do when getting around 22-30cm in height; in nature they may grow in clusters, and this helps them expand outward a little. The side its leaning toward likely has become a little weaker than the other side. As long as it looks ok it likely is: closeups might might provide more clues if anything is amiss. Might want to do something to limit the leaning though, as that might in nature be an aspect of the habitat, otherwise it may become somewhat horizontal. Maybe a decorative rock, or some stiff clear plastic to limit the leaning. Unsupported leaning can also stress the rooting and lift it loose a bit if the soil is loose, possibly resulting in it uprooting or even falling out of the container! Maybe could right it a bit by carefully planting & then placing a support based in the same container; cautiously, so as to be careful of the structures. They lean like that naturally & it helps them to spread a little and also to spread a little farther after flowering. Good interesting question and clear illustration.