What could possibly cause such a drop 2 halvings from now? Hard to imagine anything. Unless we manage to nuke the internet.
Discussion
Quantum computer
This very well could be what causes it
Quantum computers will make the hashpower higher and if it somehow can break the encryption algorythm we can always create a new quantum resistant algorythm and go back to the block before the encryption was broken. Some double spend will have occured but no one will have lost funds if stolen from the crack.