Once in the beginning of Sat Yug, there was a Rishi named Bhrigu. His wife's name was Divyaa. She used to work hard for her house. Once, at the time of Dev-Asur war, Asur got very afraid of Vishnu so all Asur came to Bhrigu Rishi. He gave them shelter, handed over his work to Divyaa and he himself went to northern part of Himaalaya to get Sanjeevanee Vidyaa. He wanted to please Shankar Jee to make Asur always victorious by getting Sanjeevanee Vidyaa.
At the same time Vishnu came there riding on His Garud and started killing Asur. He killed most of them in a few moments. At this Bhrigu's wife Divyaa got angry and got ready to give Shaap to Vishnu, that Vishnu immediately cut her head by His Chakra (Divine disc). In the meantime Bhrigu also came there after getting Sanjeevanee Vidyaa. He saw that all Daitya have been killed and his wife has also been killed. Filled with rage he gave Shaap to Vishnu - "You will incarnate ten times on Prithvi."
That is why Vishnu had to incarnate 10 times on Prithvi.