Why do planes overfly Moffett Field before landing?
Moffett Field is a federal airport (requires permission) owned by NASA and has a wide ranging number of traffic patterns/approaches available.  In answer to your question, you may be seeing what is called an "Overhead" approach.  This is a VFR maneuver quite common at military bases and usually consists of the following:


Airplane approaches the runway in use at an altitude well above the airport (2000 ft AGL or higher).  Usually this initial altitude is higher than the downwind altitude.
At some point when over the runway (over the numbers, mid-field, departure end, etc.), the pilot will break left or right and enter a descending turn that will include a modified downwind length based on the point at which the overhead break occurred.
The pilot will then continue to a base leg and ultimately a final approach (often very short) to a landing or touch and go.
This maneuver is done under VFR as coordinated with, and cleared by, the Tower.


This type of maneuver could explain, based on your question, what you are seeing at Moffett Field.  See the pictures below.

From the Moffett Airfield Operations Manual:



Example Overhead approach: (just an example, many variations are possible)