What causes a hen to be broody?

[afarrington2]

I have two hens who are broody.. I have a rooster and 3 other hens in the same area. What causes this?
Thanks!

 

[Kevin565]

I believe hormones play a part.

 

[Fred's Hens]

Instinct, nature

 

[afarrington2]

Thanks! It is really annoying it happens all of the time around here! So we always get less eggs!