Hungary's alliance with Germany in WWII was to a large degree influenced by the opportunity to rectify what it considered an injustice. To reclaim what they considered their own territory. The Hungarian government was politically and militarily allied with Germany, but not ideologically allied to the Nazis (though there were certainly individuals in power who were). To the best of my understanding, they approached their control of the Vojvodina as a way to liberate their people from foreign oppression, and they considered all the residents of the region to be their people. And while the position of Jews in that region was still tenuous, it was considered by most to be quite a bit safer to live in an area under Hungarian occupation than most anywhere else in Europe.
Which is why what happened in October 1943 took everyone by surprise.
The main massacre, however, took place in the Delvidek region's largest city, Novi Sad (Ujvidek). It was mounted in contravention of express orders from the government, which had by now received reliable information on the state of affairs in the area and regarded the army's actions as atrocities. The Novi Sad massacre was organized by the local army commander. First the city was placed under curfew and completely isolated by cutting the phone and telegraph lines. Then masses of hostages were taken, especially from among the Jewish population, which was believed to be the wealthier class. Thousands of people were arrested on false charges of "collaboration with the partisans," and their "interrogation" usually ended in death. To justify the operation the army staged partisan attacks on the local gendarmerie, after which the purportedly wounded gendarmes wore bandages on their heads and hands as ostensible proof of the violence. The troops' appetite for mayhem was stoked by a stiff drink of rum. The massacre was conducted systematically, street by street. In actions reminiscent of th atrocities committed by the Ustasha movement in neighboring "independent" Croatia, some of the victims were chased or brought by truck to the banks of the Danube, where they were herded into the swimming club two by two, forced to undress in temperatures reaching -20 degrees Fahrenheit, and were shot, with their bodies being cast into the river (after a hole had been made in the ice by cannon fire). Others were thrown alive into the freezing water. It was not until four days later that the city's governor, Leo Deak, managed to get through to his superior and that same evening the army's actions in Novi Sad were halted--although the massacre continued in the surrounding area. The toll of deaths in Novi Sad and its environs was 4116 people: 2842 Serbs, 1250 Jews, 11 Hungarians and 13 Russians.
The Holocaust - Leni Yahil, Ina Friedman, Hayah Galai. p 503.