This is a rather broad question so I'll try my best to answer but it probably won't be as organized as my other answers. You are correct that talking about background on the current war is difficult with the 20 year rule but one thing I will highlight is that the current conflict should not be viewed as a purely ethnic conflict as it had a strong political element to it. This was the case for the Ethiopian Civil War as well.

A few words on the Tigrayan peasants

For centuries, the Tigrayan peasants took pride in their cultural heritage as they viewed themselves to be the descendants of the union of King Solomon and the Queen of Sheba, heirs to the ancient Axumite empire and one of the earliest bearers of Christianity. Yet, the origin of Tigrayan nationalism is a more recent phenomenon. Until recently, a nationalist movement could not emerge because the Ethiopian state was poorly integrated and ethnic minorities had little contact with one another or the state government. State modernization, which was pursued by Haile Selassie, through increased communication and transportation broke any cultural isolation and created a basis on which nationalism could develop. Modernization also made the Amhara dominated nature of the Ethiopian Empire more obvious to the Tigrayan peasants who, as they started to travel to major cities, felt the discrimination in employment, trade and in courts. Of course, Amhara domination could be felt before as well, by peasants through contact with police, court officials, tax collectors all of whom were Amharigna speakers. Yet, in a way, Tigray was better off compared to other regions of Ethiopia. Haile Selassie pursued a vigorous centralization program by transitioning the country from feudal rule to a centralized autocracy. Yet, he was more careful with Tigray, especially in the aftermath of the Woyene rebellion. Essentially, Haile Selassie made a concession with the Tigrayan nobility which meant that Tigray would be the only region where the local hereditary nobility would continue to exist(instead of being replaced by Amhara landlords). When the Derg came to power, in order to centralized control they quickly purged this nobility(along with the Amhara nobility in other regions) to destroy the land-tenure based relations on which the authority of the old regime rested. Yet Tigrayan peasants did not necessarily felt liberated. On the contrary, With their traditional leadership eliminated, Tigrayan peasants feared they would have no one to protect them from the new Amhara elite which they were convinced had taken over the government. While, you might expect that the land nationalization program of the Derg to bring the peasantry on their side, two factors prevented this from happening:

1. Unlike the southern regions of the country, where the local peasants had been forced into a tenancy with Amhara elites after Menelik's conquest of the region, in northern Ethiopia, the peasants had enjoyed much more usufructuary rights to the land(even selling it was possible, although rare).

2)the authoritarian way the reforms were initiated as they were carried out very quickly and violently which became the source of many grievances. This was coupled with the domination of the local peasant associations by government elites which was also resented. The hatred for the Derg grew even more as the regime introduced conscription, new taxes and forced the peasants to sell their produce to the government at below market values.

All of this would allow new political movements to emerge and gain influence in the region.

The TLF and the TPLF

Yet, the rebel movements did not originate with the peasantry but with the urban intelligentsia. Among the Tigrayan intelligentsia in Addis Ababa there were two strands of thought. The first postulated that the economically poor state of the Tigray region was a product of not merely neglect but of intention to ruin the region by destroying its dynasty and unity by the Amhara elite. They argued that if not for Menelik’s collusion with the Italian colonists to break Tigray up and his periodic military forays into the region, they claimed, Tigray would not have been so impoverished. They viewed Tigray's current conditions to be a mere colony of Shewa and advocated the creation of an independent united Tigray state(that would include both Ethiopia's and Eritrea's Tigryna speaking regions). Followers of this movement would form the Tigray Liberation Front(TLF). The second strand of thought would emerge among the radical leftist Ethiopian Student Movement of the 1960s. They argued that all rural Ethiopia suffered equally from the willful neglect of an absolutist monarchical state with pretensions to modernism and pointed to the destitute regions of northern Shewa as proof. These people also saw Tigrayan identity and Trigray’s past as important components of Ethiopian identity. The followers of this movement would form the Tigray People's Liberation Front(TPLF) who abandoned the idea of independence and argued that self-determination was a right of the Tigrayan people (as it was of all “oppressed nationalities”) but that separation was an option to be exercised only if the struggle to democratize Ethiopia failed. In the early years, both organizations drew fighters not from the peasantry but from the petit bourgeois(students and teachers). However, the TPLF, heavily influenced by Maoist thought, realized the importance of winning over the peasantry to ensure victory in the revolution. This was a long and difficult process which involved years of dialogue with the peasants as well as engaging in economic reforms and infrastructure projects. This, combined with the Derg's repression allowed the TPLF to win the support of the Tigryan peasants. They also received the backing of the EPLF and eventually managed to defeat the TLF and other rival rebel groups.

The post war era

In the final years of the Derg regime, the TPLF expanded their objectives: instead of simply seeking to create an autonomous Tigray within a democratic Ethiopia, it now advocated the replacement of Mengistu's regime by a coalition of all political organizations in the country. Of course, they had no intention of sharing power with any other rebel group and instead decided to create a coalition by organizing groups that were already under its influence(and some even directly created by the TPLF). That coalition became known as the Ethiopian People's Democratic Revolutionary Front(EPDRF) made up of TPLF, the Ethiopian People's Democratic Movement(EPDM, a splinter group from the EPLF closely identified with the Amhara), the Oromo People's Democratic Organization(OPDO, a rival of the OLF created by the TPLF) and the Ethiopian Democratic Officers' Revolutionary Movement(EDORM, made up of soldiers who defected from the Derg). It was this EPDRF that marched into Addis Ababa in 1991 and they would proceed to set up a transitional government. The new government followed a policy it called "revolutionary democracy"(which was essentially adopting a Marxist political system to a free market economy) and sought to base the new nation's government on "ethnic federalism" which aims to decentralize and devolve power and decision-making to specially created ethno-regions: regarding budget, revenue collection, self-administration, the judiciary, and local development planning. Ethno-federalism was essentially the TPLF's answer to how an ethno-nationalist group could rule a multi-ethnic nation like Ethiopia. Now, I don't want to go into too much detail about the strength and weaknesses of the ethno-federalism( issues like the territorialization of ethnicity, the existence of multi-ethnic cities, discrimination of minority ethnic groups within each ethnic region) but I will mention one major issue. The fact that the modern Ethiopian government does not follow a liberal-democratic system and despite the TPLF's renouncement of Marxism even before the fall of the Derg, the EPDRF still has a vanguardist attitude towards its role in the nation's future. The government has also used the ethno-federalist system to its advantage, taking control of each ethnic region by setting up ethno-regional satellite parties closely linked to and controlled by the EPRDF. Through this system, the EPRDF(with the TPLF as its de facto leader) retained firm control over the government and this continued till the election of Abay who tried to move the country away from ethno-federalism to weaken the influence of the TPLF but I can't go into that for obvious reasons.

Sources: John Young Peasant revolution in Ethiopia,The Tigray People's Liberation Front-the go-to book if you're interested in learning about the TPLF and it's relationship with the peasantry

Gebru Tareke Ethiopian Revolution: War in the Horn of Africa- a relatively new book that primarily focuses on the military history of post-revolutionary Ethiopia but it also talks about the origins of the TPLF and EPLF

John Abbink Ethnic-based federalism and ethnicity in Ethiopia: reassessing the experiment after 20 years- This one is actually an article from the Journal of East Africans Studies so I'm not sure if you can get your hands on it, but it still provides a nice overview of the challenges of the Ethno-federalist system