@Talbot
Birds are, indeed, said to "flock" and I think Twitterers should have that word applied to them too.
I don't concur with Gromit's suggestion that there was a play on words going on. There was no story connection between a Cameron speech about the Calais migrants and a Twitter-storm about Corbyn.
However, the Mirror's use of the word was the verb form "(they have) swarmed to Twitter" - the action of behaving en masse, whereas Cameron used the word as a noun or adjective "a swarm of migrants". Using a collective noun, normally only applied to insects, amounts to likening a group of people to a collection of something-less-than-people. Hence the accusation of dehumanising terminology.
You are right about locusts and I accept that swarm has that negative connotation also. Whether bees or locusts springs to mind first obviously depends on the person reading/hearing the word swarm.
So, was the wider context of Cameron's speech that the Calais migrants were going to come here and boost our GDP by the equivalent of a small town or was it that they were going to come here and consume the food equivalent of a small town?
Can the Mirror be forgiven for thinking he meant the latter, from the tone of voice and the stress, if not the words?