Stop using it and safely isolate the supply if you can do so without entering a wet or damaged area. An unexpected start needs investigation before the appliance returns to ordinary service.
What the observation tells you
Do not lean over the opening to watch for another event or repeat power cycles to demonstrate it. Tell the engineer whether the start followed a power interruption, cabinet movement, cleaning or operation of another nearby control. These are useful circumstances, not a diagnosis.
A useful next step
Leave a clear instruction that the sink appliance must remain unused. Preserve details of any separate control accessory so both the disposer and its switching arrangement can be assessed.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.