samudram durgam asritya
badhante dasyavah prajah
These Yadavas have abandoned the holy lands inhabited by saintly sages and have instead taken shelter of a fortress in the sea, a place where no brahminical principles are observed. There, just like thieves, they harass their subjects.
The words brahmarsi-sevitan desan (“holy lands inhabited by saintly sages”) allude to the district of Mathura. Srila Prabhupada writes: “Sisupala went crazy because of Krsna’s being elected the supreme, first worshiped person in that meeting, and he spoke so irresponsibly that it appeared that he had lost all his good fortune.”