Why does water always boil over when steaming shrimp?
Foaming is the result of proteins.   For example, urine (sorry) foams when protein levels are high (persons with kidney failure can judge their urinary protein levels by how much foaming occurs).  Vegetables have no protein;  shrimp juices, plenty.  Fats counteract protein foam formation, so egg whites ( very high protein) foam nicely, but not if any yolk or other oils are present.  cream is a bit different, in that it has both protein AND fat, but that’s another story.