First off, your textbook is wrong in that the military head of the German delegation was Max Hoffmann, not von Hindenburg.

The main reason Trosky was so grating at the peace negotiations was twofold. Firstly, Trotsky was being, well, Trotsky. He harangued the Central Powers and the Ukrainian delegations with long-winded lectures on Marxist theory. His preening intellectualism made an impression on many, but it was also very grating to endure. This was one of the reasons why Stalin later found so many allies against Trotsky after Lenin's death as many found him to be insufferable and intrusive. When the Party Congress elected the Central Committee in 1919, the number of negative votes against his appointment far exceeded that of any other candidate.

Secondly, towards the end of the conference, Trotsky dropped his "bombshell," that Russia was not going to sign a peace treaty, but was unilaterally leaving the war. Trotsky announced this after a long diatribe on the natures of imperialism that many thought was just a prelude to a concession. This put the Central Powers in awkward spot of whether to continue fighting to gain a proper treaty or let well enough alone and simply take whatever portions of Russia their military could occupy. The exhausted Austro-Hungarian and Ottoman governments favored the latter course, but the Germans resumed operations and would force Lenin to sign the Treaty of Brest-Litovsk.

Hoffmann's memoir The War of Lost Opportunities has a good passage on Trotsky's undiplomatic behavior and is worth quoting at length:

With Trotsky's arrival the unconstrained- intercourse that had existed until then outside the meeting-hall came to an end. Trotsky requested that the Delegation might have their meals in their quarters, and prohibited all private intercourse and all conversations.

At the very beginning of the negotiations there was a slight collision. Mr. Trotsky seemed to consider the platform in Brest-Litovsk not large enough for his propagandistic designs. He demanded that the scene of the negotiations should be removed to Stockholm. His chief object in this demand was the wish to get away from Brest-Litovsk, the Military Headquarters in the War zone, where it would be impossible to get into direct contact with the dissatisfied elements of the Central Powers, with whose help the inflammatory portions of his speeches would be underlined and carried to wide circles of the people for propaganda purposes. Naturally the Central Powers refused this demand. Then the battle of words began between Trotsky and Kuhlmann [the German civilian negotiator] that lasted several weeks, and led to nothing. It was only gradually that it became clear to all the parties concerned in the negotiations that the chief object Trotsky was pursuing was to preach the Bolshevik doctrines, that he was only speaking to the gallery and did not set the slightest store by any practical work that was to be done. Simultaneously with his speeches wireless messages were sent to " all," inciting to revolt, to disobedience, to the murder of the officers. I protested energetically against this ; Trotsky promised to desist, but the instigating wireless messages continued to be sent. The negotiations went farther and farther from their real object and turned into theoretical discussions. Trotsky's tone became with each day more aggressive. One day I therefore pointed out to Secretary of State Kuhlmann and Count Czernin [Austria-Hungary's negotiator] that it was impossible to attain our object in this way ; that it was absolutely necessary to bring the negotiations back to a basis of facts, and offered at the next opportunity to represent to the Russians bow the position really was, and why we were assembled there. As always the Secretary of State was quite of my opinion. Count Czernin, whose nerves became worse every day, as he felt that he never came an inch nearer his object of returning to Vienna with a Treaty of Peace, had some objections to make, as he still hoped by amiability and diplomatic cleverness to get round the Bolshevik gentlemen. At last he also gave in. It was decided that at the next favourable opportunity Secretary of State Kuhlmann would pass the word on to me, I and I should say whatever I thought necessary. The opportunity came sooner than we had expected. Already the next day Kameneff, Trotsky's brother-in-law, made a speech by Trotsky's order, which made the blood of all the officers who were present rise to their heads. It was wonderfully audacious ; the Russians might have had a certain right to speak in that way if the positions had been reversed, that is to say, if the German Army had been defeated and lay defenseless on the ground, and the Russian Armies had been victoriously in possession of German territory.

Hoffmann has this to say about Trotsky's bombshell:

[Trotsky] evidently also thought that he had produced sufficient effect with his propaganda, and now sought for an opening which would enable him to bring the Brest negotiations to an end in a way that would produce the greatest possible effect. In the meeting of the 10th of February he announced, that although he would sign no Treaty of Peace, Russia would consider the War at an end from that time, she would send all her Armies to their homes and that she would proclaim the fact to all the Peoples and all the States.

The whole congress sat speechless when Trotsky had finished his declaration. We were all dumbfounded. That same evening the Austro-Hungarian and the German diplomatists had a meeting to consult. on the new position at which I was also summoned to assist. The diplomatists of both countries were unanimous in asserting that they would accept this declaration. As although no Peace had been signed the conditions of peace were established between the two countries by this declaration. I was the only one against it. We had made an Armistice with the Russians with the object of arranging the terms of peace. If peace were not concluded the object of the armistice was not attained, and, therefore, the armistice came automatically to an end, and hostilities must recommence. Trotsky's declaration was, in my opinion, nothing more than a denouncement of the Armistice.