Budgie Nesting In Soil?

Hi all, 

For sometime now I have had a female budge of mine creating nests in the soil under objects (trees/water tray) in the aviary, despite many breeding boxes being available. Prior to this behaviour she had raised 6 chicks (across 2 clutches) successfully in the breeding boxes. She has attempted multiple times to lay eggs in this nest on the ground and out of about 20 eggs she has had 2 chicks survive surprisingly. Anyhow, other females in the aviary are now copying this behaviour and nesting in the ground. Is this a normal behaviour or has anyone at least ever had their budgies do this? If not why might they be doing this?

Thank,

Amelia!