@Wonyluvr
The Somali National Movement (SNM for short) was set up by Isaaq during the late 70s in Saudi Arabia and relocated to London in the 80s, it was a movement set up to counter nepotism, corruption and chaos which Somalia endured under Siad Barre's dictatorship and to reestablish democratic system, it wasn't originally a secessionist movement, but a splinter group from Afraad a section of Western Somali Liberation Front.
Afraad had Isaaq only, but due to the rape of an Isaaq woman near Wajaale and her teenage brother shot the two WSLF fighters dead, they executed the boy and some of his family members, the Afraad section then killed 14 Ogaden WSLF fighters for harassing the local population.
The Derg regime launched a massive campaign to clear the WSLF in the 1980s, WSLF managed to escape into northern Somalia, and Mengistu began supporting the SNM afterwards, the resulting war between SNM and WSLF was important in Ethiopia's strategy to exploit clan divisions within Somali society, SNM with the support of Ethiopian army forced WSLF out of its bases in the north and by 1985 the WSLF ceased to exist.
Somali Republic was set up for failure from the start, Northern Somali remained unchanged due to the British not being able to centralize power within the region, so instead of enforcing colonial systems, they opted to stick to the traditional institutions that have governed the region for centuries if not for the last Millennium, meanwhile the Italians introduced colonial style governance in the south, by centralising power and removing traditional governance.
The cracks appeared early after the failed war with Ethiopia in the late 1970’s. The north had a huge influx of Somali refugees from the Somali region, most being Ogadens, due to the scarcity in water tensions were high between the locals and the refugees, the Siad Barre regime made local militia exclusively made up of the refugees(this was made to fight alongside the WSLF) then started to commit injustices against the Isaaq, especially since it was in their territory, alongside the lack of development in the North which also affected Warsengele and Dulbahante (Darood), the Isaaq, the Gadabursi and the Issa.
In the mid 80;s after the closure of the Saudi market for Somali cattle exported, which consisted of 80% of the Norths trade, concentrated even more power to the south, following the peace accords signed by Siad Barrie and Mengistu to stop supporting rebels in the late 80’s, the SNM took action by attacking the cities of Burao and Hargeisa, the government responded with mass executions, heavily artillery bombardment and aerial bombing, refugees fleeing the combat who headed to the Ethiopian border were not spared, the SNM itself was not innocent either, killing several hundred civilians in the north.
The situation is more complex, and I didn't include the civil war in SL between the Isaaq in the 90's, one of the reasons the north didn't descend into complete chaos is due to traditional institutions that remained in the north.