Anyone know what it means if rooks don’t sleep in their rookery

We have a rookery near our house.  That is a small area of trees were rooks nest.  For years and years and years the rooks have roosted in the trees.  What a noise they make and what a nuisance they can be when I put the bird food out

When dusk was coming in I saw this flock of rooks flying away from these trees.  They flew over the fields.  I watched them until the disappeared in the distance.

Why did they do this?