Since the secp256k1 curve order is a prime number, every point on the curve except the point at infinity is a generator.

Nothing is known about how the designers of the curve selected this specific generator.

There is, however, a telltale sign that points to its construction. If the selected generator G is multiplied by 1/2 (ie, by the multiplicative inverse of 2 modulo of the curve order), the resulting X coordinate is an exceptionally low number. This most likely means that G was created by selecting that X, finding a corresponding Y in the curve, and then doubling the resulting point.