What caused my meringue to fall after adding cocoa?
It was timing.  If you want to make chocolate meringues, add the cocoa powder to the liquid egg whites, and then whisk it all together at once.  

Beaten egg whites are bubbles.  Don't add things to an already-formed meringue, as they will just cause the bubble structure to collapse.

Ever sprinkled sugar or cinnamon powder on a cappuccino?  Notice how the milk foam immediately collapses?  The same thing happened to your meringue.