Please compare the shape of 1st one and others. The 1st one has a smoth surface and cylindorical body.did the first one recover from surgery?
When you watered to a cactus, inner body pressure is increased. Since a cylindrical shape can not change the body shape, maximum stress is formed on 2 location of surface (skin) which is called as hoop stress. The cactus on photo has 2 vertical cracks. The other one is located opposite side.
Since the bottom of the crack is very sharp, the crack is developed gradually deeper and wider.
You can see the cracks on grafted asterias often but it may be occured on own-rooted asterias also.
Frank