Q: Why Thyagaraja took Sanyasa?

A: It is belived that Sri Rama appered to him and said, you have yet to take another birth before reaching moksha. If however you take sanyasa now you can reach moksha because Sanyasa is a rebirth. Hearing it Thyagaraja embraced sanyasa in the prescribed manner and called himself as Nadha Premmananther and he reached God while singing the Lord's praise.

Q: Why Thyagaraja took Unchavritti?

A: Things were to get worse as Sarabhoji II surrendered his kingdom to the East India Co. and retired as its pensioner. In the absence of any regular source on income, Thyagaraja was forced to resort to Unchavritti, a form of holy mendicancy which has been given great importance in the Shasstras as it destroys ego and teaches the seeker the supreme quality of humility. Dressed in the traditional attire of the Unchavritti bhagavathar, and with the clappers in his hand, he would wander around the streets of Thiruvaiyaru with his disciples, singing bhajanas and collect the food given willingly by the householders They would collect enough for a week at a time and no more.
Despite religious sanction Thyagaraja seems to have been unhappy with this arrangement. In a rare kriti [Ennalu tirigedi...] he pours out his anguish to Rama. "How long am 1 to wander like this, begging for all my wants, deluding others with my pooja and japam and forgetting your worship? O lord, do not abandon me".