There are lots of theories around. The best I've heard is from Luke Piper, and American living in China and professionally refurbishing screens.
In short, for the iPhone 5 series screens, the main source was a "leak" in the Apple supply chain. Apple busted a Chinese group that was responsible for the leak and it dried up. That happened at the same time that the manufacturing businesses had switched over to the iPhone 6, which created a sustained dip in supply.
Luke's forecast is that ever inventive China will figure out how to capitalize on this price increase and we will see supply increase after Chinese New Year (Feb), which will gradually drive prices back down.
The spike in iPhone 6 assemblies (in Luke's opinion) was due to refurbishing folks worldwide who realized that the 6s uses the same LCD. Therefore, it makes sense to hang onto their stock and refurbish cracked 6 screens into 6s screens and wait to sell when the market is very high just after the new phone releases.
This too will gradually come back into equilibrium as the 6s screen price drops.