My pet theory on this (where no LD is present) is that these sudden bursts of development are really just an exhibit of perfectionism, because the gifted child is reluctant to display the skill until they are convinced they can do it perfectly.
My DD didn't demonstrate reading new material on her own until she was 5, though we were fairly sure she already had the ability. She already knew her phonics, a number of words by sight, and she was reading signs, the TV screen, etc, so it was not much of a stretch to think she could put it all together for a Berenstain Bears-type book.
I finally challenged her on it by telling her all the ways that she was ahead of me in her development at that age, except one... I was already reading aloud to my mom when I was 4. The week didn't end before she read a new book aloud to her own mom. That success gave her the confidence to show what she could do, and shortly thereafter she was reading aloud to her pre-K class.
