We use the term juvenile flakies because it is most accurately describes what's technically known as a second fear imprint period. Juvenile flakies are apprehension or fear behaviors that are usually short lived. They're caused by temporary calcium deficiencies and hormone development related to a puppy's periodic growth spurts.
The timing of event isn't clearly defined as the first fear imprint period, and it coincides with growth spurts; hence it may occur more than once as the dog matures. Even though he may have been outgoing and confident before, your puppy now may be reluctant to approach someone or something new and unfamiliar, or he may suddenly be afraid of something familiar.
Fear of new or unfamiliar has its roots in evolution. In a wild pack, after the pups become 8 to 10 months of age, they're allowed to come on a hunt. The first lesson they have to learn is to stay with the pack; if they wander off, they might get lost or into trouble. They also haft to develop survival techniques, one of which is fear. The message to the puppy is "if you see or smell something unfamiliar, run the other way." Apprehension or fear of the familiar is also caused by growth spurts. At this point in a puppy's life, hormones start to surge. Hormones can effect the calcium uptake in the body, and, coupled with growth, this can be a difficult time for the growing puppy.
This is an excerpt from "Dog Training for Dummies"