Obviously this is after the initial 'puberty' phase of your dreadlings, past the initial 2 year locking up process that seems to take forever when you're in the midst of it! But then when you're hitting the 3rd & 4th year anniversaries of your locs, you'll start to notice them growing thick & fast. The answer to this is two-fold: you aren't shedding any hair & these hairs tangle into your dreads, giving depth & thickness to your lovely locs. This is a good thing! The other reason is lack of breakage of the hair. Hair doesn't tend to break if it's encased inside your dreadlocks, protecting it and matting up, as opposed to single strands that can snap easily. If you're not attacking your hair with curling wands, straighteners, brushes, hair dryers, etc, the hair doesn't break as easily, allowing it to grow long & lush.
If your hair is damaged or breaking, there's a few things you can do to help your locs grow and grow fast.