Why do Communist societies that we've seen tend toward authoritarianism and dictatorial-style arrangements?

There is no single accepted answer among academic historians or even among leftists. As one might expect, for such a politically charged and relevant question it becomes really difficult to separate history proper from politics.

One reasonable way to approach the question is to look at what the leaders of the revolution thought and how they preceived or justified authoritarianism. It's also important to understand their political experience and context.

A good starting point I think is the late 19th century, when the Second International, an association of primarily Marxist parties across Europe, suffered an internal crisis between reformists and revolutionists. At that point, the revolutionary character of Marxism was not yet agreed upon among its followers, and within the parties reformist Marxists tended to hold sway. Unlike modern social democrats, these reformists still (if only nominally) held the ultimate goal of overcoming capitalism; the major disagreement was if this could be achieved by working entirely within the bourgeois state. This question of reform or revolution is discussed with some more historical context in the revolutionary Rosa Luxemburg's eponymous pamphlet, written at the height of this conflict. Eduard Bernstein's works give the reformist perspective.

(As an aside, Rosa Luxemburg was a revolutionist in Germany and Poland who collaborated extensively with Lenin. She would later disagree tactically with Lenin during the Russian revolution but, as I believe, they had no fundamental doctrinal differences-- one can expect that Luxemburg's comments about reform and revolution are broadly the same as Lenin's)

As Europe approached the First World War, the reformist tendency within socialist parties would deepen and even take on a nationalistic character. By then, the German Social Democratic Party (SPD) had become the dominant socialist party within Europe, having achieved significant labor reforms and boasting a membership in the millions. This, combined with the threat of invasion from a deeply reactionary Russia, compelled German reformists to associate socialism with German patriotism, and national defense with the defense of socialism (ironically, a similar trend would occur in France against Germany, which many Frenchmen perceived as a reactionary threat against their republic). Revolutionists, on the other hand, tended to have an internationalist outlook, believing that only an international revolution against imperialist warfare could defend socialism.

When WW1 broke out at last, the SPD (and most other European parties) voted in favor of war, beginning a period of Burgfriedenspolitik wherein the SPD abstained from strikes and other subversive activities against the war effort. The revolutionists and few reformists who disagreed with this policy splintered away from the SPD into the Independent SPD, the Spartacist League, and eventually the German Communist Party. Notably, Lenin's party in Russia also opposed war by majority, foreshadowing the dominance of revolutionists in the upcoming struggle.

This is all to say that the events leading up to WW1 and the Russian revolution precipitated huge, seemingly irreconciliable divisions between the reformist and revolutionist factions of the European socialists. This conflict would take on its most bloody form in the 1918-19 Spartacus uprising at the end of the German revolution: once in power, the now wholly reformist SPD would brutally crush their revolutionist counterparts as they attempted to establish worker's states. Remember again that these were former comrades who, decades ago, would have relied on each other's cooperation.

So what does this have to do with authoritarianism in Russia? The leaders of the Russian revolution were internationalists, and had counted on the victory of the German revolutionaries and, hence, the victory of a sweeping revolution across the world. The hope was that, since Germany was among the foremost industrial powers of the world, it could provide material necessities and alleviate the stresses of war, in turn allowing for demilitarization and democratization across both Germany and Russia. This didn't happen-- again, the German revolutionaries were massacred by reformists. It's possible that the Bolsheviks miscalculated, or simply that the Bolsheviks had no other choice but to push forward and hope that Germany would turn around. Probably both. In any case, the ascendant Bolsheviks were left in a very, very difficult situation: they were in charge of a war-weary nation with hostile states (particularly now Germany) on all sides and powerful counterrevolutionaries within. Realistically there was only one thing they could do besides capitulate-- dig in, and use the full power of the state to survive for as long as possible.

Rosa Luxemburg summarizes this impasse pretty well in The Russian Tragedy (which is short but extremely useful in understanding the attitudes of communists at the time -- a must read!):

The awkward position that the Bolsheviks are in today, however, is, together with most of their mistakes, a consequence of basic insolubility of the problem posed to them by the international, above all the German, proletariat. To carry out the dictatorship of the proletariat and a socialist revolution in a single country surrounded by reactionary imperialist rule and in the fury of the bloodiest world war in human history – that is squaring the circle. Any socialist party would have to fail in this task and perish – whether or not it made self-renunciation the guiding star of its policies. ... Such is the false logic of the objective situation: any socialist party that came to power in Russia today must pursue the wrong tactics so long as it, as part of the international proletarian army, is left in the lurch by the main body of this army.

As an example of "wrong tactics," Trotsky discusses in a report that the early Soviet worker's militia could not confront the vastly better-equipped and better-trained German army, nor even the relatively more experienced armies under White control. As a result, Trotsky was compelled to professionalize the army, institute harsh drafts, and even incorporate captured White officers at gunpoint just to win the civil war. Both he and Lenin recognized that these measures were counter to the principles of the revolution but were nonetheless necessary if the revolution were to survive at all.

As we now know, neither Germany nor any comparably industrialized country underwent a successful communist revolution. The Soviets would remain isolated long after Lenin's death, and Stalin would further entrench authoritarianism-- what started as emergency measures became standard procedure when the emergency never went away.

For subsequent revolutions, the "fruit from a poisoned tree" situation you described is sort of right-- these revolutions drew explicitly from Lenin's revolutionary measures and, sometimes less explicitly, from Stalin's entrenchment. One might even say that they were all part of a single, broader revolution, and can't be treated as isolated cases; this is contentious, however. Later on certain countries like Cambodia and North Korea would spin off on their own and abandon communism even in name, but since they faced the same problems of political isolation, they had at least an excuse to remain authoritarian.

It's probably not true that failure was inevitable, which would ignore the later history of Trotskyism and other oppositional forces in, most notably, the USSR and the China. But I don't want to spin off into counterfactuals-- things could have happened differently, but they didn't, and that's the question you're asking.

In summary:

Pre-WW1 politics caused a deep rift between reformist and revolutionary factions in European socialism
This led to the (short-term) success of the Russian revolution and the failure of the German revolution, as reformists were much more powerful in the latter country
Without German support, the Russian Bolsheviks had to enact emergency authoritarian measures to remain afloat
Later, the static international situation allowed Stalin to entrench authoritarianism
Future revolutions would emulate the Bolsheviks and in turn face the same problems of political isolation

Note again that this is just a single perspective, taking into account mostly primary sources from the leaders of the Russian and German revolutions. Still, this is an important point of view I think, and the one I'm most familiar with.