When you consider the recent large volcanic eruptions - Pinatubo in 1991, Mount St Helens in the 80's, Kilauea in Hawaii and Etna in Italy both of which are at it all the time, to name a few, there are many more - doesn't it strike you as odd that there's never been a blanket grounding of air traffic before for...a-hem... 'ash'?
Now I'm not one for conspiracy theories (no, no, come on I'm not
that bad...

) but just supposing 'someone' wanted complete freedom of airspace over Northern Europe for a couple of days to carry out some type of clandestine activity, the like of which we can only speculate on, away from the prying eyes of commercial pilots and passengers, wouldn't this be a perfect cover story...?
But, nah, it's probably kosher and they can't be arsed to hose all the planes down when they're covered in shit.