What is the most major factor affecting deviancy?

Bid’ah is the most common and critical area which requires understanding because it has become accepted as bid’ah hasanah [good].

A heretical innovation in religion. Prophet (ﷺ) said, “He who does something which is not in a matter of ours will have it rejected.” [1]

The deviation in practices into Islam is forbidden and not in other practices

• Original Innovation – something which has no origin in religion.
• Circumstantial Innovation – There is some basis in religion but is carried out differently with changes.

Example: One can raise hands in prayer for rain. It has become a common practice to make dua after congregational prayer. When it becomes an established practice, people feel it is incomplete without it.

Example: Saying Salaam after prayer as habitual custom or saying sadaqa Allahul 'azeem after reciting the Qur'an.

There is no innovation in religion.

Reference:

[1] Bukhari, no. 127/3, Muslim, no. 132/5, Abu Dawud, no. 4606