After outlining the Anglo-German naval rivalry and the tariff reform debate, Alfred Gollin, one of the few historians to discuss the subject in any depth, has this to say about the origins of the 1909 phantom airship scare:
This was the intense condition of Britain affairs when the Prime Minister, H. H. Asquith, made his announcement about the government's new aeronautical policy in the House of Commons on 5 May 1909. His speech produced a curious and remarkable result.
People in several parts of England now began to see airships in flight, in places where it was impossible for such aerial vehicles to be.
It's true that it was right about this time that phantom airship sightings took off. However, some of these took place before 5 May, some as far back as March. Moreover, the contents of Asquith's speech (which was quite short and hardly deserves the name) were not exactly sensational and seem unlikely to have caused much apprehension. He had two main points to make. The first was that 'The Government is taking steps towards placing its organisation for aerial navigation on a more satisfactory footing':
As the result of a Report made by the Committee of Imperial Defence, the work of devising and constructing dirigible airships and aeroplanes has been apportioned between the Navy and the Army. The Admiralty is building certain dirigibles, while certain others of a different type will be constructed at the War Office Balloon Factory at Aldershot, which is about to be reorganised for the purpose. The investigation and provision of aeroplanes are also assigned to the War Office.
The second was to announce the formation of a 'Special Committee' under the presidency of Lord Rayleigh and the chairmanship of H. T. Glazebrook to oversee 'investigations at the National Physical Laboratory and for general advice on the scientific problems arising in connection with the work of the Admiralty and War Office in aerial construction and navigation'.
It could be that it was the way Asquith's announcement was reported that was the trigger. But while the major newspapers did report it, again there doesn't seem like there was much to excite the general public. Most press reactions that I've looked at treated it as a welcome, if overdue, development, and expressed hopes that Britain would now be able to catch up to Continental standards in aviation — not only those like the Manchester Guardian which were in political sympathy with the government, but also those which were not, like the Standard, the Globe, and even the Manchester Courier, which by 1913 had definitely decided that the aerial defence of the nation could not be entrusted to the Liberals. It's true that The Times and the Observer did criticise the makeup of the Rayleigh committee (mainly on the grounds that there were very few members with practical aviation experience), but even so there was no suggestion of immediate peril.
So I'm sceptical. But I'm also sympathetic. It's natural to seek some definite cause of these puzzling events — I do it myself: I think successfully in the Australian case in 1918, with the report of the Wölfchen's flight over Sydney; less so in my 4th year thesis, when I suggested that the outbreak of the First Balkan War was somehow responsible for the Sheerness incident. Why did some people see mystery airships in March 1909? Why did a lot more start seeing them in May? Why did they stop seeing them by the end of the month? The last is actually relatively easy to explain: the scare collapsed under its own weight, as too many airships were being reported to be credible and the press became sceptical. By the same token, the press was certainly crucial in the expansion phase, by reporting on the growing phenomenon and suggesting to people that there really were airships flying around at night. So it's finding the initial spark that is the real problem, and generally there isn't a satisfactory one to be had. I think it's essentially random. People see strange things in the sky from time to time. Sometimes they think they're airships, because what else could they be? Usually they are ignored, even if they tell somebody. Sometimes, though, the reports are picked up and amplified by the press, which is when the scare proper begins. There's no single ultimate cause; it's more the vibe, the popular understanding of aviation. To an extent this process is irrational, then; which makes me think that maybe Asquith's announcement could have been one of the triggers after all.