There are many threads on plucking here that you can read through. One of the first suggestions you will find is that a visit to the vet may be in order to rule out and health related causes. If that is cleared then you will want to think about if any big changes have happened in the house recently that may be causing stress or upset.
Do you bath your bird? May be itchy skin.
How much time is your bird out?
The causes, can be many.