Radak: Not only will it place the crown of their pride on the land. It will even trample it after it is on the ground! This is a metaphor for their great lowliness. Teramasnah is [feminine] plural, for each of the crowns, i.e. everything that they are haughty about. Ateres in singular, for the Klal The same applies to "Ki Sikrenah Milchamah" (Shemos 1:10).

Malbim: Placing the crown on the land is like removing it from the king. Being trampled on the ground is totally removing kingship via exiling them from their land. These correspond to the hail and flood, respectively. The hail (killing) made the crown of kingship fall. Galus totally trampled the crown; it cannot be put on another king, since all were exiled. Sancheriv came three times against Efrayim. The first two times, a remnant remained, but the third time, they were trampled to extinction.