Depending on the exact ratings and circuit used, either of the following:
1)
Transition from DCM to CCM. In DCM, the PWM cycle is short enough so that while the current rises during on-time, the stored energy in the inductance fully discharges during off-time, to zero. Each cycle starts from zero - easy to understand. There is no carryover state.
In CCM, the current rise during on-time is more than the current fall during off-time, so the current doesn't reach 0. Instead, current value is stored from the previous cycle. On each cycle, it gets higher and higher.
You need current feedback, instead of dumb PWM. The feedback can be a simple "terminate the cycle when current limit is reached", or a PI control loop controlling the PWM value, based on current measurement. Either way, it uses actual current measurement, hence understand the already stored energy.
2)
Saturation of the motor inductance. When the current increases, the effective inductance drops. After a certain point, the inductance drops very quickly. This means, the same on-time will cause much higher current rise, if the current is already high.
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Stalling the rotor exposes the winding inductance because the back-EMF voltage is simply zero. The circuit can be analyzed as a typical buck converter.