I have a classifier with a heavily imbalanced dataset (1000 of each negative label for each positive.)

I'm running a GradientBoostingClassifier with moderate success (AUC .75) but the curve has this strange look:

[![enter image description here](https://i.stack.imgur.com/8E0y9.png)](https://i.stack.imgur.com/8E0y9.png)

Any good ideas on what would cause the curve to have this behaviour?