It just looks like the cactus experienced a difference in environment, and therefore a difference in growth characteristics. I'm not sure how long you've had the cactus, but most plants are grown in more favorable conditions than we can provide at home. So sometimes after a plant gets settled into it's new home, it will change it's growing habits to better suit it's new environment. It looks healthy to me though, so I wouldn't worry about it. Could also be slight transplant shock, if it was re-potted around the time the change took place. Also, that soil looks a little too water retentive for cactus. It might be a good idea to plant it in something faster draining, like a 50%-60% pumice mix. The container should have drain holes in the bottom, to allow excess water out. If the soil stays wet for too long, the cactus will rot and die. It really needs to be in a container with drain holes, and I would plant it in a faster draining mix, while at it.