What you describe is the textbook definition of a habit: routines of behavior that are repeated regularly and tend to occur subconsciously. These are triggered by some external (say, being behind the driver's well of a car) or internal (say, being upset) stimuli. This activates chunks of procedural memory, which attempts to carry out a task that was usually associated with the stimuli (keeping the car on the road; eating a cake).
Although chunking is best understood for factual memories, a similar procedure is believed to apply for how procedural memories become a habit. Obviously, this is a highly researched topic, mostly because of the connection to addiction.
A final minor point is to be careful with terminology. Subconscious is a vague term that should typically be replaced by unconscious or preconscious.
Wood, W., & Neal, D.T. (2007) "A new look at habits and the habit-goal interface." Psychological Review, 114(4): 843-863. [pdf]
- A relatively well know article that shows a model of how habits are shaped and interfere with or aid conscious goals.
Barnes, T.D., Kubota, Y., Hu, D., Jin, D.Z., & Graybiel, A.M. (2005) "Activity of striatal neurons reflects dynamic encoding and recoding of procedural memories." Nature 437, 1158-1161 [pdf]
- A look at the neurobiology of chunking of procedural memory and habit formation in rats. Stresses how easy it is to reactivate old habits.
Graybiel, A.M. (2008) "Habits, Rituals, and the Evaluative Brain." Annual Review of Neuroscience 31: 359-387 [pdf]
- Reviews evidence for the involvement of basal ganglia–based circuits in procedural chunking and habit formation. Show that habits form for both overt behaviors (say, backing your car out of the driveway) and cognitive behavior (say, general route planning).
Dehaenea, S., Changeux, J.-P., Naccachea, L., Sackura, J., and Sergent, C. (2006) "Conscious, preconscious, and subliminal processing: a testable taxonomy" Trends in Cognitive Sciences 10(5): 204-211. [pdf]
- Explains the difference between unconscious, preconscious, and subliminal and how to test for them.