German airports have been on heightened alert for a few days. One of the suspects (a known islamic extremist, FWIW) had been triggering a security alert in France (at Charles de Gaulle Airport) just before he came over to Stuttgart. Flats and homes have been searched in connection with that, though no one was arrested or implicated in a cunning plan.
Maybe (but that's highly speculative) there's a connection? Trying to shut down multiple airports across Europe? Unsubs scared away by security action here in Germany (and France) and tried it on the isles as long as it is easily possible to cross over?