Once they went to Vaikunth to see Shree Hari. Hari's  gatekeepers (Paarshad), Jaya and Vijaya, didn't know them, so they asked them as where were they going? Sanakaadi Muni got very angry at their question and said - "You are not worthy to be the gatekeeper of Shree Hari." and gave them Shaap that they should be born as Raakshas. In the meantime Shree Hari came out and requested them to pardon His gatekeepers. 

At this they said - "Hey Hari, You yourself may decide their Shaap." And Shree Hari reduced their Shaap to (1) they will not forget Brahm Vidyaa in all their lives, (2) and they should be killed only by Shree Hari, (3) they should get Mukti from that Yoni (species) after three lives. So they were born as Hiranyaaksh and Hiranyakashyap in their first life in Sat Yug, as Raavan and Kumbhkarn in Tretaa Yug, and as Shishupaal and Dantvakra in Dwaapar Yug,. So to give Mukti to Jaya and Vijaya Vishnu Himself had to take Avataar in all the three Yug. To give Mukti to Hiranyaaksh, He appeared as Varaah Avataar, To give Mukti to Hiranyakashyap He appeared as Nrasinh Bhagvaan. To give Mukti to Raavan and Kumbhkarn He had to come in human being form as he had a Var that he wouldn't be killed by any Devtaa, Gandharv, Asur or Raakshas, so He appeared as Raam. And to give Mukti to Shishupaal and Dantvakra He appeared as Shree Krishn.