A Compression Event is when you have so many rock shards lying around that the rock re-absorbs them and makes future shards more compact. Your runners still collect the same amount, but it appears in less quantity. Drones and Zygnorps can collect these heavier shards without any change. Be aware, a compression event means the rock will slowly re-absorb any loose shards from now on and this rate increases with more shards currently in your pile.
To reach a compression event, Simply harvest more shards than you collect. A quick way to do this is to purchase the "Cocoa Cafe" and purchase the "Breaktime upgrade from "The Express" (Runner building).