Magnetic Solenoid

In most cases on solenoid valves, the magnetic "core" is the steel core around which the coil is wound. It is not made from a solid mass, but rather from a stack of thin stamped plates make of grain oriented silicon steel that are stacked on each other to form that shape, then connected together to create that mass. Small coils use rivets or screws to keep them together, larger ones use welds. In your photo, you can see the 4 holes in the core for these screws or rivets. If a screw or rivet breaks, the core partially separates under the magnetic stresses and warps the plates, making it unreasonable to repair (compared to the cost of just replacing it).

Why did that happen? Bad luck mostly, nothing is 100% reliable. But one application issue that can do it is rapid and repeated chattering of the coil circuit, meaning whatever is energizing that coil is not giving it a continuous flow of current to keep it pulled in. That can also happen if the proper voltage is not applied, because the act of pulling in causes a voltage drop, which weakens the magnetic field, which makes it let go, which allows the voltage to increase and the coil to attempt to pull in again, which causes a voltage drop again and this cycle repeats until something breaks or a fuse blows.

So bottom line, after replacing it, thoroughly investigate your coil circuit and control devices for any problems and if there are none, it was just bad luck.