I guess if the fumes were heavier than air they could have pooled around floor level, possibly knocked him unconscious eventually with a fridge or something starting up and sparking to cause the explosion but that must have been one hell of a lot of fumes. Perhaps it could have caused a secondary explosion on the gas supply but if that's been ruled out already (ish) who knows.
Remember this;
http://news.bbc.co.uk/1/hi/england/1876342.stm