I've read a lot about how the POUM was not exactly on good terms with either the Republican government (including the Communist Party) nor Trotsky himself, and I know that the POUM was declared illegal more-or-less because they were blamed for the May Days in Barcelona.
What I don't really understand is why was the POUM singled out? I know the anarchists were blamed too, but even they seem to have gotten off easy compared to the POUM. (And honestly, it seems like it would be easier to place the full blame for the May Days on the anarchists, not their barely participating allies, the POUM) Is it because the POUM were anti-Stalinist and opposed to the mainstream Communist Party in Spain? If so, why did the Communists care so much about the POUM? I don't see why the POUM's dissent would be such a big deal to the Communists. Were they really that afraid of POUM encouraging a revolution?
Apologies if I have some of my facts wrong.
This was mainly a method of propaganda used by the USSR. The POUM was anti-stalinist and very critical of both Communism in the USSR and its growing power in Spain. As Bolloten explains, "in accordance with the tactic used by Stalin at the Moscow trails of amalgamating all opponents under a single label, the Communists denounced the dissidents of the POUM as Trotskyist agents of Franco, Hitler, and Mussolini." At the time, the USSR was the dominant force in international communism so this sort of thing actually had a lot of power.
The PCE and the POUM actually represented the same ideology in reality. Their conflict was, in a way, separate from the rest of the Popular Front's infighting because their main difference was the POUM's lack of affiliation with the Comintern due to their opposition to Stalin.
Take, for instance, the first round of Moscow Trials in which Grigory Zinoviev and Lev Kamenev were executed. The POUM immediately held protests against this. (Paraphrasing the words of Victor Alba because accurately copying from a book is annoying) "The CNT, Socialists, and Republicans did not understand the significance of what had happened; the CNT especially treated them as if they were family quarrels."
The POUM were also more drawn to loosely siding with the Anarchists because the PSUC and PCE were both making concessions with the republic in order to further Stalin's goals at the cost of a revolution. Though a big generalization to make, you could say that both the Anarchists and the POUM would take any revolution over none. Though, self interest does win out as the Anarchists agreed to remove the POUM from the Generalitat. In exchange, the CNT was allowed to maintain power over the revolutionary militas in Catalonia and the PSUC would withdraw candidates from the new cabinet. However, the PSUC still gained 3 positions running as UGT candidates.
With the POUM seemingly out of the picture the PSUC moved to hamstring the Anarchist. With pressure from the PSUC the Generalitat soon gave control of the regional army to the state devastating both the POUM and the CNT.
On April 7th the PSUC and UGT launched their plan for victory against the Nationalists which included:
Creation of a unified army under central government authority
Nationalization of war industries
Creation of a unified police force under Generalitat (read PSUC) authority.
Handover of all arms to the government
This contradicted the interests of the POUM and the core beliefs of the Anarchists so it naturally caused a bit of outrage. A series of political assassinations targeting the government bought an air of foreboding. About the atmosphere, Orwell writes "There is going to be trouble before long."
The POUM openly took the side of the Anarchists. Though they favoured strong government, they didn’t like the PSUC in control of it.
So when the May Days happened the POUM members were there (some would argue reluctantly) fighting alongside the Anarchists, firmly taking a militant stance against the Popular Front.
Hope this kinda answered your questions.
Sources:
Orwell, George. Homage to Catalonia. First America Edition ed. New York, New York: Harcourt, Brace, 1952.
Payne, Stanley G. The Spanish Revolution. New York: Norton, 1970.
Bolloten, Burnett. The Spanish Civil War: Revolution and Counterrevolution. London: The University of California Press, 1991.
Orwell's "Homage to Catalonia" ( http://www.amazon.co.uk/Homage-Catalonia-Penguin-Modern-Classics/dp/0141183055/ref=sr_1_1?ie=UTF8&qid=1430121984&sr=8-1&keywords=Homage+to+Catalonia ) is very good on this, as is Anthony Beevor's "The Battle for Spain" ( http://www.amazon.co.uk/Battle-Spain-Spanish-Civil-1936-1939/dp/0753821656/ref=sr_1_1?ie=UTF8&qid=1430121887&sr=8-1&keywords=Spanish+Civil+War ) and, of course, Hugh Thomas' "Spanish Civil War" ( http://www.amazon.co.uk/Spanish-Civil-Thomas-Hugh-2003/dp/B00E3FVPDO/ref=sr_1_4?ie=UTF8&qid=1430121947&sr=8-4&keywords=Spanish+Civil+War+Thomas ). Essentially your suggestion "Is it because the POUM were anti-Stalinist and opposed to the mainstream Communist Party in Spain?" is the answer. The Republic was pretty much being run by and for Stalinist Communists, and although they may have disliked the Anarchists and other political groups, the Stalinists claimed to represent all communists, as such a communist group that was opposed to Stalinist communism was perceived by them as a worse enemy than the Fascists.