Force of habit
Definition 1 of 1
Pro Tip 1/3Unconscious Action
The phrase often refers to actions done without conscious thought, as if on autopilot. 
She found herself checking the mailbox every day out of force of habit, even though it was Sunday.
Pro Tip 2/3Change Resistance
'Force of habit' illustrates the difficulty people face when trying to change a long-standing routine. 
He kept reaching for the light switch that was no longer there, a force of habit hard to break.
Pro Tip 3/3Inadvertent Errors
The phrase may be used to explain mistakes made because someone defaulted to a habitual action. 
Typed my old password out of force of habit and got locked out of my account.