This is my own conclusion based on what I have read and seen.
The conflict between Shia and Sunni started over the successorship after Muhammad. The Shias use Hadithes, which also include some Hadithes in Sunni collections to prove that Ali was the chosen successor after Muhammad. The Sunnis, in order to reject and fight back, reject these Hadithes, and to do that, they have cruiticisid Shia collection of Hadithes, such as Biharulanwar or Al-kafi.
Then, in return and for retaliation, the Shias often call the Sunni Hadith narrators and collectors Liars. This is a well-known conflict. And I doubted anyone who is aware of Sunni and Shia, would disagree. You can easily find info on this.
Here are some info
Shia–Sunni relations - Wikipedia
Now, to add more to this, after the Manifestation of the Bab, the Babis, and Bahais used Hadithes, specially from Shia collections to prove that the Bab matches the signs of the Qaim (aka Mahdi) described in Hadithes. Now, at this point on the Shia leaders started to say that such Hadithes are false Hadithes, even though it is in the Shia collections. This is what I have experienced on this RF, when discussing with Shias. So, my conclusion is, the sects tend to use this excuse that since the Hadithes cannot be proved to be authentic, whenever they say something that they don't like, they simply reject it, even when it is in their own Hadith collections. The result of these efforts, has been resulted in the Quranism sect!