Turbo pumps have parts spinning at thousands of RPM, which means the blades in the pump have a great deal of energy. A turbine blade crack can lead to a fracture which will fling the failed turbine part into the pump. The best case would be for the pump to fail in a contained way, either by the piece jamming something or ripping bits of it apart, which would lead to a loss of power in the rocket which may or may not cause an abort. The worst case scenario (and more likely given the energies involved) is an uncontained failure of the pump which will perforate the rocket in multiple places, including other pumps, fuel lines and tanks, which will lead to a catastrophic explosion.
Turbine failures in rocket motors have happened. In the Space Shuttle program there were contained pump failures where after a failure in testing:
A post-test inspection revealed that a first-stage turbine blade had
broken off and inflicted significant damage to both turbine stages.
Figure 17 shows the damage to the first-stage wheel. The engine was
shut down safely with no other engine damage. Two weeks later, on
December 1, 1977, Test 901-147 on Engine 0103 experienced a similar
failure at slightly above 80 percent power level. This time the damage
was more severe. The turbine blade debris caused the rotor to seize
up, resulting in the cessation of fuel flow and very LOX rich
operation. Major burning throughout the hot gas system followed; but,
although significant damage was sustained, it was contained within the
engine with no external burnthrough.
More recently an Antares rocket exploded just above the pad when a turbine failed in flight:
The failure began in one of the Antares rocket’s two AJ26 engines,
when a spinning rotor inside a liquid oxygen turbopump contacted
another component inside the powerplant, triggering an explosion,
according to engineers.
This resulted in:
Once a rocket lifts off any kind of abort will require the activation of the launch escape system for the crew to survive, which is not a scenario to be taken lightly as there's a great deal of risk involved. If a satellite is destroyed on launch it's a shame but you can build an identical one, if a person is killed you don't have that option, which is why there's this scrutiny.