Mysterious creeper-like explosions

When I play on my Hard difficulty survival world, part of the ceiling of my blacksmith's keeps missing blocks.

What's interesting is that it looks like a creeper explosion, but the fact that I never saw any creeper there, and that it only happened in that spot, made me think that the cause may be something else.

The wood is catching on fire and burning from the lava.

If there is an adjacent flammable block (e.g: wood above), lava can create fire in air blocks in this shape:

You should be able to move your roof up one, make it out of non-flammable blocks, or put blocks such as torches in all the spots that fire could occur in order to block it.