Campfires can blow embers into new areas that start a fire.

Or people who are burning rubbish on their property, etc

Discarded cigarettes butts

Sparks from machinery, trains breaking, etc

Electrical arcs (e.g. if a power line gets blown down or shorts out in strong winds)

Arson

If you have a bunch of flammable stuff in a pile and the ideal conditions for a fire to start and grow it doesn't take much

Edit to add: The lightening also doesn't need to be the typical lightening most people would associate with also having a thunderstorm attached to it. There's also "dry lightening" which doesn't need rain to happen. (I can't really ELI5 what causes dry lightening because I don't fully understand it myself)