Too much butter in your brownies can make them turn out greasy and mushy.
Butter is an emulsifier and it makes cookies tender. It also adds in the crispy-around-the-edges element. Adding too much butter can cause the cookies to be flat and greasy. Adding too little butter can cause the cookies to be tough and crumbly.
Other culprit for baked pastries that turn out too flat:
Added too much fat or sugar.
Baked at a low oven temperature.
Used room temperature/soft dough.
Used a warm pan.
Tip! See links below to learn tricks on how to rescue common pastry problems:
Betty Crocker: Cookie Troubleshooting Guide
CakeSpy: Cookie Capers: Fixing Botched-Up Butter in Batter
BBC GoodFood: Baking SOS by Richard Burr
MNN.com: What's wrong with my cookies? A troubleshooting guide