Sale of indulgences
One of the practices that Martin Luther denounced was that of Tetzel or the sale of indulgences by the clergymen, which also helped increase the control of the Church in the German States (another phenomenon that Luther was vehemently against).
What indulgences entailed
"Through granting indulegences, the Church supposedly reduced the time of a soul would have to suffer punishment in Purgatory."
"Those seeking the salvation of their souls did not purchase God's forgiveness (which could only be received in the confessional) but rather cancelled or reduced the temporal punishment (such as the pilgrimages, prayers, or charity) required to atone for their sins."
The problem of the idea of "indulgences" was that most people perceived it as a way for the "rich" to get an unfair advantage by using their wealth to get "an immediate entry to Heaven for oneself or one's relatives" (which was not how indulgences worked).
Renaissance's "humanitas" as the prelude to the Reformation
One of the important effects of the Renaissance was that it reshaped the way people saw the world (everything from their relationships with themselves but also the institutions dominating the political, social, and religious life). Besides putting morality and ethics under scrutiny, individuals started to pay more attention to the corruption of the Church and its fundamental tenets, such as the "idleness of the monastic life." While the progress was slow at first, the ideas became even more pervasive among humanists, which ultimately paved the way for the Reformation: challenging the practices and doctrine of the Church.
"For, like the Renaissance, the Reformation was in some ways the work of humanists moving beyond what they considered the constraints of Church theology."
On asking fundamental questions
Fundamentally, the Reformation was started when people began "questioning fundamental Church doctrine and the nature of religious faith."
"The resulting reform movement, culminating in the Reformation, shattered the unity of the Western Christendom."
Moreover, once one starts asking fundamental questions and those questions (and solutions) reach the right mass of people, it is tough to ever go back (without transparent reform or a whole new institution).
"But Luther could not have survived the ban of the empire if his influence had not already spread, convincing many that through Luther they had now discovered the true Gospel."
Rejection of the Church and the new (reformed) religions
"Reformers rejected the pope's authority and the Church doctrine itself. The movement for reform, or of 'protest,' came to be called the 'Reformation.' It led to the establishment of many 'Protestant' denominations within Christianity. The followers of the German priest Martin Luther became Lutherans, while those of the Frenchman Jean Calvin in Switzerland became known as Calvinists."