Froglotion: Earth stake can be a source of ignition. So it doesn't really matter if they say so or not. It's the same deal with fibre, there was a notice at some point saying it is considered a source of ignition too. Even though the power at which fibre here is run isn't considered high enough to be a source of ignition. It's all about butt covering and covering situations where a fault could cause a problem.
The whole reason for the exclusion zone is that if the cylinders vent, there is clearance. I've never seen a cylinder vent. Or ever heard of it happening. So it's all to cover than one in a million situation.
Possible gas leaking from the bottle, or hoses cracking? I have seen a lot of cracked gas hoses from age and exposure to elements.