I am in the moment mailing with the author of clan cleansing in somaliaDarod "flocked" to Siad Barre AFTER they had been targeted by USC.
The experience of being killed and driven out of Mogadishu by the USC was what so angered all the Darod sub-clans that they put aside their political differences and united to fight back.
Had the USC not systemically killed innocent Darod people, like the anti-Barre, pro-democracy activist Dr. Abyan, Barre would have received very little support from Darod, who were fed up with him as much as anybody else (A lot of the anti-Barre insurgent groups were Darod, like SSDF, SPM, etc).
Even Marexan was split into two factions, with one faction refusing to back Barre.
USC was ruthless and shortsighted and very incompetent. They made a bad situation incomprehensibly worse.