The inductor, output capacitor, and load resistor form an under-damped resonant circuit (suggest you look up resonant circuits if you don't understand that).
The initial transient is just the overshoot and settling of that resonant circuit to the step-input from the start of the simulation.
In a normal buck regulator there is a feedback loop with compensation to cancel the effects of that output resonance so there is no output overshoot or ringing in its operation.