Certain reasons why they bite:

-Seeking attention

Due to :

-Hungry

-Thirsty

- Something is wrong

or

- Teeth Itchy

Things you can do to prevent :

- Take the hamster out and comfort it

- Sometimes it could be related to the other animals around it, like sensing something is wrong with the other

-Put hamster chews for itchy teeth

- Check cage for any missing, or faults

Hope this helps :)