This is such a good question, have my up vote.

As u/Njordsvif rightly stated, there was a significant increase in the study of German history and traditions by a group of romantic writers who were informed by a period of German literature known as Sturm und Drang (Storm and Stress) from 1795-1810. Sturm und Drang originated from Weimar, led by writers Goethe and Schiller, and sought to combat the enlightened ideals of Aufklärung (Enlightenment), a prominent part of the system of governance in central Europe during the middle of the eighteenth century. They attempted to do this by showcasing the power of nature, human emotion and individualism as greater than the rationalisation and logic championed by French philosophers. Further influenced by Johann Georg Hamman, whose distrust of reason and logic was in distinct contrast to Kant, Sturm und Drang was a form of proto-romanticism, which attempted to classify the culture of Germany, with the hope that the ideals of the proto-romantics would go hand in hand with a kulturenation and dispel the French thought and theory exemplified by Voltaire and followed by the Prussian King, Frederick II. Originally, this period of German literature was not intentionally written to be the antithesis of Aufklärung, but merely an intellectual dialectic between two schools of thought, though Hamman as influenced by Hume, placed significant weight in God as a divine being.

The rise of Romanticism and the romantics coincided with the study of a ‘German’ past and was used as a direct counter to the French Revolution. It was an attempt to rebut the middle class thinkers, who now controlled France and who actively sought to promote the revolution as the natural progression of enlightenment thought. Edmund Burke, the most famous counter-revolutionary agreed with many of the leaders of the revolution, and believed enlightenment did inform and encourage the violent destabilisation of the ancien regime. Within Germany such thought was explored by a group called the Jena Romantics, led by the two brothers Friedrich Schlegel and August Schlegel. This group looked to dispel the rhetoric of the revolution and its catch cry Liberté, égalité, fraternité by placing it as an ideal counter to what it meant to be ‘German’ through plays, novels, pamphlets and poems.

First and foremost these writers were actively seeking to counter a political thought that would displace them and their status as men of knowledge, wealth and honour. In turn, lumping them in with the peasants, bourgeois, professions and artisans. A social class they had no inclination or desire to a part of. In order to confront the encroaching ideology of the French Revolution, now making it’s away across the Rhenish Palatinate and southern Germany with the armies of France, Friedrich Schlegel and his brother looked to classify the ideas of the revolution as ‘other’ and thus counter to central Europe and Germany. Yet, in order for there to be an ‘other’, the Jena Romantics had to create the idea of Germany and what it meant to be ‘German’.

Up until the beginning of the French Revolution, the idea of being German and Germany was a purely linguistic label used to cover the hegemony of the language in central Europe. To the vast majority (most probably 95%), the Holy Roman Empire was an archaic administrative and undemocratic system, which provided them with no social cohesion. The local particularism and regional patriotism of the small kingdoms of the Reich and the inability for most to travel, negated any genuine consensus on what it meant to German. From the very highest of the class echelon to the very lowest, the direct connections between King and his subjects and the will of the dynasty trumped any thought of ‘Germany’. Loyalty was owed to the sovereign who governed an individual’s local region, whether that was state, province or parish. Thus, the romantics hoped to mobilise those who shared a language in support of ancient privileges, which separated the aristocracy, the clergy and those they gave patronage from the rest of society, who according to the ideals of the revolution-deserved equality.

Schlegel and his brother looked to build on the ideas of a German kulturenation as the foundation of their German nation. The Jena Romantics drew upon on the shared similarities of traditional German folk stories, an idealised version of the medieval period, where the Teutonic kingdoms embodied German masculinity and the tribes of Germania, which were a direct opponent to the classic civilisations of Rome and Greece studied by enlightened thinkers, to create a shared national consensus. An important source for the writers of this early ‘German’ history was the historical and ethnographic work found in Tacitus’s Germania. It is here that a definitive outline of ancient Germany was provided and particular attention was paid to the landscape, foreign to the roman ‘other’ used to the Mediterranean and its coast. In Germania, Tacitus lavishes attention on the tribes of Germania and depicts them as a noble and worthy people, counter to the soft and lazy romans he continuously critiques. Of particular importance was the religious practices of Suebi and their Druids (druids maybe of a Celtic tradition, but to the Romans they were synonymous with barbarians) who worshipped under a Sacred Oak grove. In chapter 39 Tacitus states: ‘At a set time all the peoples of this blood (Suebi) gather, in their embassies in a wood hallowed by the auguries of their ancestors and the awe of ages. The sacrifice in public of a human victim marks the grisly opening of their savage ritual'. It is from this passage that the Romantics were given the strongest possible symbol highlighting the myriad of old German tribes united, one which would be translated into the present day. It stuck out for these men for two reasons. First, the worshipping under a Sacred Oak grove (it is inferred as an oak grove as Tacitus make mention of it as a tree foreign from Italy) appealed to the Romantic epistemology, which emphasised human activity condition by nature in the form of custom and usage. Second, the gathering of embassies as one to participate in a religious practice highlighted a time when the German people were united. These two points, coupled with the oak tree as consistent across central Europe, became a clear motif for the Romantic writers and was used to consistently mark the kulturenation of the people.

As a follow-up question or potential suggestion for a rabbit hole to go exploring - could it have any connection to Irminsul or Yggdrasil, the ancient Germanic pagan holy trees?