A few points that came to me while discussing this with hubby:
First: Honor was starting to get strange. His Intent was taking over his personality, and causing problems, such as caring more for any oath, than for actual honor.
Second: The Knights were squabbling quite a bit amongst themselves. Possibly even close to starting a civil war between Orders.
Third: I don't see how the spren didn't know what their Knights were going to do.
Fourth: Pattern offered to sacrifice himself to save Shallan pain, because he thought she hated him. And if he died, then she could bond another spren, one who hadn't killed her parents.
Fifth: Their ancestors destroyed their previous planet, then came and took over Roshar, and brought a rather nasty Power with them.
Sixth: We have this quote from Nale:
So my conclusion is multi-fold: The Knights had lost their way, and when they found out about this, it hit them while they were vulnerable and they broke. This might have been encouraged quite a bit by Nale and his Skybreakers, since one of the epigraphs stated that a lot of the infighting was between the Skybreakers and the Windrunners.
Also, I believe that they had to have conspired with their spren to sacrifice themselves. Pattern is willing to sacrifice himself to spare Shallan some pain. It wouldn't be a huge reach to think their spren would be willing to sacrificed themselves to keep from destroying the world. By killing their spren, they ensured that spren would not form bonds with humans for the foreseeable future, thus preventing future Radiants from being created.