Parasitic oscillations are usually generated due to:

Parasitic oscillations are unwanted, spurious RF signals generated within an amplifier stage. They typically occur when the physical layout of the amplifier components—such as stray capacitance between wires and the natural inductance of component leads—creates unintended resonant circuits. If the conditions within the amplifier allow for feedback at these accidental resonant frequencies, the amplifier will oscillate independently of the intended operating frequency. These self-generated oscillations can rob power from the main signal, cause splatter, and even destroy amplifier components. They are not caused by mismatched transmission lines or excessive drive, which cause different issues like high SWR or flat-topping. Furthermore, they are distinct from harmonics, which are mathematical multiples of the fundamental frequency created by non-linear amplification, rather than independent self-oscillations.