Blocks catching fire

I was building a rail system to bring ores and stone from my mining shaft, when I noticed a block on fire. It was at first an obsidian block. I put it out and went back to the rail system. I saw the block was on fire again, so I put it out, and removed the block and removed some of the lava underneath.

I noticed later that a stone block was on fire on the other side of the lava pool. Everytime I put it out, it would catch fire again a few minutes later.

I have since removed another portion of the lava underneath and haven't seen the flames again yet.

These flames were NOT created with a flint and steel. I can only assume that the flames were created by the lava underneath.