Details from An Abridged History of Lyrian about it:
Fearing an alliance between his greatest rivals, Zokar first sent his forces south to eliminate Certius, the lesser threat, and succeeded in the endeavor. The maneuver became known as Zokar’s Folly, because an alliance between Certius and Eldrin was highly unlikely, and the operation gave Eldrin time to prepare. In anticipation of the impending battle he created the drinlings.
Prior to the climactic battle, Orruck and Maldor fell out of favor with Zokar, an ironic turn of events considering that it was Arastus who eventually betrayed Zokar in exchange for the right to become Eldrin’s first and only apprentice. In the end Zokar was forced to face Eldrin in single combat, and the legendary duel is widely considered the End of the Age of Wizards.
This passage means that Zokar thought that Certius and Eldrin would combine their forces to stop him (which was unlikely). Because he believed this, though, he tried to take away the lesser threat, Certius, and he succeeded. But this was a mistake, because the alliance was unlikely, and Zokar's actions gave Eldrin time to prepare. This led to Zokar's eventual demise.