Sh. Saalih Al-Fawzaan said

1) Imported Methodologies

If we discard the methodology which our pious predecessors were upon and seek to introduce methodologies from here and there, we will become divided and every group will have its own methodology which differs from the others and every group will claim the others are mistaken.

2) Listening To Liars And Their Likes

Secondly, from the causes of separation and differing is listening to liars and slanderers, and to the false rumours and propagation which the weak in faith, hypocrites and biased ones propagate from within, those who do not want us to be united upon a single faith and religion. This is why it is compulsory upon us to verify narrations an to not make haste, as Allaah (swt) had ordered us to verify narrations (see Quraan 49:6).

3) Finding Fault With Muslims And Having Evil Suspicion

Thirdly, finding fault with the Muslim who has not committed a sin is impermissible and evil suspicion is impermissible. All of this is a prohibition against finding fault with the Muslims and from listening to those who backbite and slander them. For this reason Allaah made backbiting impermissible. And the worst of all is the one who moves between the students of knowledge and callers to Islaam in order to spread corruption and cause within the Muslim community and to incite feelings of hatred between them.

4) Abandoning The Muslims

The fourth cause of separation is: abandoning the Muslims, and abandonment means desertion and distancing. Meaning a person distancing and cutting himself from a person and no speaking to him.