Lightning will do that. One thunder storm may have hundreds of strikes. Now think of a band of storms. Even if only a small % of strikes start fires, the result of multiple thunderstorm bands blowing through can start many fires. Especially if the forest is very dry.

A few weeks ago Cliff Mass had a post on his blog about fires in Oregon and Washington. Scroll down in it to see a lightning strike map from several bands of storms that blew through during one 24 hour period back in June. Even if only 1% of those strikes caused fires, it's still a lot of fires! Dry conditions and thunderstorms are a bad combination in the West.