Further background that confirms that Mas’ud (transliterated differently below) instigated the attack as a direct assault on Dostum.

"... while Dostum had remained neutral up until then, Massoud decided, in early December, to attack the river port of Sher Khan Bandar (Kunduz province), which was defended by fighters from Jumbesh. Why this attack? Assem Akram, in his book History of the War in Afghanistan, offers several explanations. First, he mentions the rivalry between the two men, which had been exacerbated by their struggle for the control of the northern provinces. Dostum had managed to supplant Massoud in six provinces (Baghlan, Samangan, Jowzjan, Balkh, Sar-i Pul, and Faryab), which made it impossible for Massoud to join forces with Ismail Khan (who was fighting against Dostum for the control of Maymana then) in the west.

Besides, Dostum had got closer to President Rabbani, to such a point that the latter entrusted him with the presidential guard when he visited Saudi Arabia. Rabbani was probably trying to counterbalance Massoud’s military power by going closer to another military heavyweight. Ahmad Shah Massoud saw the danger and so decided to take the offensive in Sher Khan Bandar to oblige Rabbani to choose between him and Dostum. However, under the pressure of Rabbani, he gave the town, after having conquered it, to Jumbesh. From then on, according to Assem Akram, who lived these events from the inside, Massoud and Ismail Khan decided to expel Dostum from the north of the country ..."

source: site.voila.fr/pour_asm/englishbiographie.htm