At one point, the headline was changed to read "Syrian migrant 'behind German blast'". But at that point, the fact that there was a German blast was widely know

At one point, the headline was changed to read "Syrian migrant 'behind German blast'". But at that point, the fact that there was a German blast was widely known; the unconfirmed portion was that a Syrian migrant was behind it. So if anything, the headline should have been written "'Syrian migrant' behind German blast". But the BBC didn't do that, because all indications are that BBC journalists pepper their headlines randomly with quotation marks. Randomly peppering headlines with quotation marks absolutely is not standard practice. Most American newspapers don't do it.