Why?
On reason is because some believers are not focusing on Jesus Christ. They are not spending their time doing His work and are spending too much time watching other believers when there are those who don't know Jesus at all that they should focus on sharing the Word with.
On the other hand there is something bigger at work. The fact is there ARE false teachers and prophets in the world and it is not only God's instruction to share the truth it is a natural result of being saved by Jesus. God's perfect truth and His light exposes darkness and sin and sin does not like to be exposed so the Gospel divides people. Scripture tells us this flat out and this situation makes for an Us vs Them feeling sometimes.
Those in cults and/or following false teachings don't want to think they are wrong so they are resistant to the truth. Those who have been called to share the truth are going to share it and it makes for an often tense situation.
I believe there would be a lot less tension and a lot of folks getting saved if some people would learn to share God's truth in a more loving way, but then sometimes even the most compassionate sharing of the Gospel can be met with down right anger from someone who's been deceived and is living in darkness.
Sin, false teachings and false prophets are a reality and it's a reality we are directed to confront with God's perfect truth and when that happens the conviction starts and often so do the fireworks.
2 Timothy 2:14-21
14 Of these things put them in remembrance, charging them before the Lord that they strive not about words to no profit, but to the subverting of the hearers. 15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth. 16 But shun profane and vain babblings: for they will increase unto more ungodliness. 17 And their word will eat as doth a canker: F3 of whom is Hymenaeus and Philetus; 18 Who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some.
19 Nevertheless the foundation of God standeth sure, F4 having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity. 20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour. 21 If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.
1 Timothy 1:3-4
3 As I besought thee to abide still at Ephesus, when I went into Macedonia, that thou mightest charge some that they teach no other doctrine, 4 Neither give heed to fables and endless genealogies, which minister questions, rather than godly edifying which is in faith: so do.
Jeremiah 5:29-31
29 Shall I not visit for these things? saith the LORD: shall not my soul be avenged on such a nation as this? 30 A wonderful F25 and horrible thing is committed in the land; 31 The prophets prophesy falsely, and the priests bear rule by their means; and my people love to have it so: and what will ye do in the end thereof?
Jeremiah 29:8-9
8 For thus saith the LORD of hosts, the God of Israel; Let not your prophets and your diviners, that be in the midst of you, deceive you, neither hearken to your dreams which ye cause to be dreamed. 9 For they prophesy falsely unto you in my name: I have not sent them, saith the LORD.
Matthew 7:15-20
"Beware of false prophets, which come to you in sheep's
clothing, but inwardly they are ravening wolves. Ye shall
know them by their fruits. Do men gather grapes of thorns,
or figs of thistles? Even so every good tree bringeth forth
good fruit; but a corrupt tree bringeth forth evil fruit. A
good tree cannot bring forth evil fruit, neither can a
corrupt tree bring forth good fruit. Every tree that
bringeth not forth good fruit is hewn down, and cast into
the fire. Wherefore by their fruits ye shall know them."
2 Timothy 4:3
"For the time will come when they will not endure sound
doctrine; but after their own lusts shall they heap to
themselves teachers, having itching ears;"
2 Peter 2:1
"But there were false prophets also among the people, even
as there shall be false teachers among you, who privily
shall bring in damnable heresies, even denying the Lord that
bought them, and bring upon themselves swift destruction."
1 John 4:1
"Beloved, believe not every spirit, but try the spirits
whether they are of God: because many false prophets are
gone out into the world."