Because the assumption that there's no danger, coupled with the fact there's now oxygen inside the tank, much more likely to be near the flammable or explosive ranges.

When full or in use you expect it to have fuel in it and usually take more care (obviously the welder isn't), and if a leak catches fire it then turns into a torch, until the tank overheats and bleve's (very unlikely that you don't notice the torch before the bleve)

The big risk of full is, you might have a steady leak that stays in the area, and then you ignite it somehow. But the likelihood is very low that it would get to the flammable or explosive ratio in open air like this.