minecraft-java-edition
I was out exploring and I ran into an area where all the trees were burning–the leaves were gone, it was trunk pieces that will never burn up.
Now, I have had a wildfire get out of hand some time ago but I don't think it burned anywhere near this area and there are plenty of unburned trees between my wildfire and this patch.
Do wildfires happen on their own?
The answer is a short, simple YES. According to the Minecraft WIKI
While individual mobs can spawn inside signs, pressure plates, and other non-opaque blocks, packs cannot. They need a completely empty block in the center. Since a pack spawns all at one level, there needs to be some air blocks right above a flat surface to allow mobs to spawn on it, and more air blocks will always allow more spawning.
To give you an example of a mob pack, this is it:
Example of a mob pack spawning. The 41×1×41 spawning area is shaded blue (not to scale). The yellow figures represent the actual positions that mobs could spawn in after checking the environment. Note that the mobs can spawn inside torch and ladder blocks. But they can't spawn on top of glass because it's not opaque. The red cube is the center of the pack, which must be an air block, but the blocks above and below it can be anything.
According to the Minecraft Wiki page on zombies:
In the Overworld, zombies spawn in groups of 4 at a light level of 7 or less. Zombies have a 5% chance to spawn as a zombie villager and a 5% chance to spawn as a baby zombie. Baby zombies have an additional 5% chance of spawning as a chicken jockey.
This means that they have a 5% chance to spawn at any point in the night.
Best Answer
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...