Wood variants such as stripped logs , regular logs and planks seem to catch fire when not even directly touching a on fire block 

When a netherack block is set on fire using a flint and steel , if a wood variant is in a certain radius (around 6~7 blocks) those blocks catch fire 

Even if the fire is encased in non flammable blocks it still occurs 

However this only happens if the wood blocks are placed first and then the netherack is set on fire , if for example the netherack is already on fire and then the wood is placed it has no effect 

If a on fire block is again set on fire then the wood placed previously catch fire