One of two possibilities, really - Here in the UK agricultural diesel is tax-free (very low tax?) and in order to prevent normal people using it, it has a red dye added ('red diesel'). There's a market and an illegal industry in processing this stuff to remove the dye - so that it's less risky to use/much easier to sell to the non-farmer - as chances of getting caught much less if fuel isn't red! So sometimes these processes lead to dodgy fuel, essentially. Running it through cat-litter, was one amateur solution apparently!!
So secondly - there's the organised-crime thing of getting proper diesel, and diluting or adulterating it with something much cheaper than diesel - whatever necessary - in order to increase its volume. This is then sold to small petrol station outlets, who can be tempted by this nice source of cheap diesel, which can be sold at normal (extortionate) pump prices.
Often when these fuels destroy engines - the connection is hardly ever made, with which fuel station one might have happened to use in the recent past.
That's laundered fuel!