Most likely the bird had runny droppings that got stuck to the area and if the droppings dried and were stuck there, the bird may have pulled out the feathers when trying to remove the stuck droppings. It is not unusual for a bird to have runny droppings for a few days when being brought into a new home but should be checked by an avian vet if it does not clear up. Take a look at this link
Pasting of the Vent
Pasting of the vent is the official name for what the rest of us call "oh no, my budgie has droppings stuck to its bum!":eek: ;) It is important to keep our eyes on all parts of our birds, including their vent, to be sure everything is in good working order. Pasting of the vent (when...