Could smoke allow arcs to form?
Interesting problem. Smoke can conduct electricity.
From the research here (first Google link btw.), "A surprising result was that the conductivity between parallel plates remained high although the optical density in the smoke chamber dropped drastically, indicating that there was very little smoke in the air."

The research also states that high voltages promote the growth of 'soot bridges', further strengthening the assumption that yes, smoke can cause arcs between high-voltage traces.