Naturally occuring wildfires in Minecraft?
Solution 1:
As you explored, you triggered a chunk to be created. Minecraft decided to put a lava fall in that chunk. The lava spread on the ground. The trees became close enough to catch fire as a result and the fire quickly spread out...
Solution 2:
Now with the weather, lightnings can hit a tree and start a fire. But it's rare because the rain will extinguish it.