I bet you had something with the EGR get funky. Like the EGR valve stuck open or something. It is supposed to recirculate exhaust gasses for emissions purposes. However exhaust gasses are hot, so it would make sense that if something were to stick open, it could easily melt the plastic. That is just my guess, and could totally be off. I will say I haven't seen that one pop on the site before.
It also could have been some type of electrical issue. Any CEL before the incident?
I will have to go and look at where the EGR enters the intake on my truck tomorrow. Different motor but should be a similar location. I think the EGR is the piece on the right side of the melted part in your picture. If you want to know for sure, it is the metal tube that comes off of one of the manifolds. I want to say it is the driver, but I could be wrong. It goes straight up from the manifold to the top of the motor though, so it is hard to miss.