The issue is long running.

The region was disputed since the Armenian-Azerbaijan war which fought in 1920 for Karabakh control, which led to the Shusha Massacre (1920) against the Armenians.

Separate to this there were the March Days (1918) where Armenian Dashnaks, with the Russian Bolsheviks, took part in the killing of Azerbaijanis in Baku. This was followed by September Days (1918) where Armenians were killed by Azerbaijanis in Baku.

The issue of secession from Azerbaijan, and unification with Armenia, started then from the beginning when the Karabakh region was assigned to Azerbaijan SSR by the Soviets (decided July 1921).

The Armenian response was to continuously petition and advocate, thinking this kind of influence with the Soviets could change the situation. (it didn't)

See for example the pettition to Khruschev in 1964 which has specific economic complaints.

The Armenian population of the Azerbaijani SSR has been subjected to chauvinistic polices creating extremely unfavorable conditions of life. At the inception of the autonomy, certain positive steps were undertaken for the development of industry and agriculture in the region. Subsequently, however, every enterprise has been thwarted, and established institutions have either been inhibited from functioning or have been transferred to regions inhabited by Azerbaijanis. On the other hand, demands were imposed upon our region which were exacting and beyond the limits of our capacity. They resulted in such desperate acts as our beings forced to extricate wool from our beds, bedcovers and pillows in order to meet the quotas imposed upon us.

There were also cultural issues. Schools didn't have Armenian textbooks, TV did not show Armenian, and there was an explicit effort to move Azerbaijani's and Turkic people in to the region (“By doing this, I tried to increase the number of Azeris and to reduce the number of Armenians.”-Heydar Aliyev). When Armenian football team played, it was not allowed to cheer. Leadership chosen by for the region were at times patsies.

However with the later period of growing openness of the Soviet Union this lead to open demonstrations for secession, and this angered many Azerbaijanis. This lead to the Kirovobad, Sumgait and Baku pogroms against Armenians within Azerbaijan proper. (1988-9)

Gorbachev also responded to these demonstrations and demands with an iron fist, via the ethnic cleansing of Operation Ring which was conducted using Azerbaijani soldiers (1991)

Despite all this for quite a while Armenians and Azerbaijanis did live side by the side, not just in Karabakh but in both Armenia and Azerbaijan proper; That is until the pogrom and the conflict flared up.signing