Is it possible they are gathering together in preparation for migrating for the winter? It reminds me of a quote from Watership Down;
“Before such people can act together, a kind of telepathic feeling has to flow through them and ripen to the point when they all know that they are ready to begin. Anyone who has seen the martins and swallows in September, assembling on the telephone wires, twittering, making short flights singly and in groups over the open, stubbly fields, returning to form longer and even longer lines above the yellowing verges of the lanes-the hundreds of individual birds merging and blending, in a mounting excitement, into swarms, and these swarms coming loosely and untidily together to create a great, unorganized flock, thick at the centre and ragged at the edges, which breaks and re-forms continually like clouds or waves-until that moment when the greater part (but not all) of them know that the time has come: they are off, and have begun once more that great southward flight which many will not survive; anyone seeing this has seen at the work the current that flows (among creatures who think of themselves primarily as part of a group and only secondarily, if at all, as individuals) to fuse them together and impel them into action without conscious thought or will: has seen at work the angel which drove the First Crusade into Antioch and drives the lemmings into the sea.”
Or for a more factual account, the RSPB page here.