A typical rocket ascent trajectory is relatively steep because of the desire to get out of the dense part of the atmosphere quickly, both to reduce losses due to aerodynamic drag and also to minimise structural loads on the rocket that would be imposed were it to accelerate to a high speed in denser air. While this maximises the payload you can orbit with a given rocket, it has the consequence that if the booster fails during ascent, it leaves the payload on a relatively steep up-and-down ballistic trajectory, which means that when it re-enters the atmosphere, rather than skimming the edge of the thin upper atmosphere as it would returning from orbit, it slams quickly into denser and denser air which subjects the spacecraft to high deceleration forces and also imposes a shorter but more intense load on the heat shield.
An earlier post here, “The Wild Suborbital Space Flight of Soyuz 18a” (2022-01-13) discussed a Soviet launch in 1975 in which the booster failed and the Soyuz descent module plunged back into the atmosphere from an apogee of 192 km, subjecting the crew to a deceleration of 21.3 gravities, which is on the edge of human survivability. Both cosmonauts were injured. The commander recovered but never flew again, while the flight engineer flew on three subsequent Soyuz missions.
The Boeing Starliner / Atlas V deliberately flies on a flatter trajectory than optimal, “wasting” some of the potential performance of the booster so that should a launch abort happen at any point prior to normal spacecraft separation, the capsule will be on a trajectory that does not subject the crew to g-forces in excess of those considered safe. This comes at a cost: the Centaur upper stage of the Atlas V must be configured with two RL-10 engines to support this trajectory, which roughly doubles its already princely cost.
Another safety feature of the Starliner launch profile is that the Atlas/Centaur does not quite put the spacecraft into orbit. Instead, it leaves the Starliner on a trajectory which intersects the atmosphere on the other side of the globe. Starliner then uses its own maneuvering engines to add the small additional velocity to circularlise the orbit. This means that if the maneuvering engines were not to work after capsule separation from the booster, the crew gets a “free return" to Earth just by waiting half an orbit.
These safety features are superior to those provided by the SpaceX Crew Dragon, which uses a steeper ascent trajectory and whose Falcon 9 second stage places the capsule directly into a circular orbit. Whether this is worth paying around twice as much per seat for the Starliner is up to NASA to decide.