It happened during a rough spot for him as a failed boxer, among other things, per Wall St. Journal. At his mother’s urging, he got very serious about being a Muslim.

During this turmoil, his mother [then living in the
U.S.] encouraged him to turn to Islam. “I told Tamerlan [Tsarnaeva, the dead
marathon brother] that we are Muslim, and we are not practicing our religion, and how can we call ourselves Muslims?” Mrs. Tsarnaeva said. “And that’s how Tamerlan started reading about Islam, and he started praying, and he got more and more and more into his religion.”

Relatives and friends say they saw a shift in the young man. Neighbors noticed that the parties stopped. “I’m telling you, something turned,” said Mr. Vasquez. “And it was dramatic.”

Tamerlan wasn’t the only Tsarnaev who was changing. His mother grew more religious alongside him. She quit her job at the spa and started doing facials in her home, saying she didn’t want to work on men. “I started reading and started learning, I started reading with my Tamerlan,” Mrs. Tsarnaeva said.

Her sister-in-law, Maret, said she was startled by the transformation. She recalled having a Skype conversation with Anzor, while he was in Makhachkala, and spotting Zubeidat in the background covered in a veil. “We’re not used to seeing her like that because she used to wear high heels and a low dress,” Maret said.

He got religion and went overboard. Major religion story here.