Some hens just do that for no reason (submitting maybe?). Some do it to be picked up. Some are horny lil betties and want to have a rooster come along. I have boys, a bunch of boys, and I have a hen who adores me. The feeling is mutual lol she is so cute, she follows me all round the yard and wants to be pet and fed and loved. When I want to pick her up, she squats for me. I know she gets a lot of rooster attention so I think she does it so I can pick her up easily.