Centrifugal force throws the liquid to the outside of the volute leaving the air at the eye of the impeller. Operating off the BEP can break the pump shaft because the force is always in the same direction while the shaft is turning. This has the affect of flexing the shaft twice per revolution. In many cases you can easily exceed the endurance limit of the shaft material. The stresses imposed in reverse bending are cumulative.
Most fatigue failure occurs in one million cycles or less.
At 1750 rpm you get 2,520,000 cycles per day. If a 300 series, stainless steel shaft is running in a fluid containing chlorides, the shaft is subject to chloride stress corrosion problems that can be another cause of shaft cracking and breakage. Slurry pumps have some features that make them different than chemical pumps. The pumps are more massive Looser tolerances. The clearances are more open.