One source I read said that it wasn't a lighter, but a lighter refill cannister. This makes more sense to me than a plain lighter, as a refill cannister stores a good deal more gas than a lighter, and under pressure. This tracks with the way the dryer door bursts open prior to the explosion, seemingly indicating that the cannister lost integrity and releases its compressed contents. The increase in pressure within the dryer causes the door to open, the mixture of gas with oxygen hits the right concentration and boom.
It's interesting that the dryer pops open and a few clothes spill out a second or so before we see the fire. Presumably it's the influx of oxygen that causes the fireball to erupt, but what causes the door to open?
Might be over-pressure due to the bursting open of the fuel.
A man in Galicia (Spain) forgot a lighter in a pocket, filled with gas. When in the dryer it caused an incredible explosion that would have killed him, saving his life by seconds.
I cannot believe it was actually a lighter; I blew up a fair number of lighters during my pyromaniac youth and never witnessed anything remotely like the explosion in this video. You get a pop and a flash - it's not especially dangerous. This looks like it had to have been a much larger mass of fuel inside a pressurized container.
I was expecting a pop, maybe a blown out door, but I was NOT prepared for all that destruction. It definitely seems like something with more flammable liquid than a lighter was in that dryer.
How do they know it was 'forgotten'?
Probably applying Hanlon's Razor which is "Never Attribute to Malice That Which is Adequately Explained by Stupidity."