The time constant comes from the coil and capacitor values.
Unstable operation results if the switching frequency is close to the resonant frequency.
Here is a simulation to portray the effect of the resonant frequency.
The scope traces begin by showing several cycles of normal running.
Then a switch was closed to bypass the transistor. This creates a resonant LC tank circuit. The waveform is much longer than the switching waveform.
The LC tank frequency is suppressed while the converter is running normally.
However when power is first applied, the sudden shock induces LC resonance for a few cycles.