The probability of a lightning downstrike starting a fire depends on if the strike has a component called a continuing current.  This was news to me until I read it on the Wildland Fire Assessment System site. I knew that positive strikes were more likely to start a fire than a negative strike, but now I know why that happens.

All positive strikes have continuing current, but only 20% of negative strikes do. The probability of a downstrike starting a fire depends on if it has continuing current (or the duration of the current) and how receptive the fuel is to igniting, with fuel moisture playing a prominent role.

From a posting this week on Wildfire Today.