One cause of lens fogging is when warm moist air comes in contact with a cold surface, the water held in the air as vapor condenses into tiny liquid drops as it cools against the surface.
Lens fogging generally happens when taking a camera from a cold/dry environment into a humid/hot environment. It can also happen when taking a camera from a hot environment into a cold environment, but that is not as likely.
Think about ski goggles, or glasses. They tend to fog when you come into a warm room after being out in the cold. Spots optics also fog due to our breathing (our breath is warm/moist) and is more likely if one is in a cold environment (skiing for example).
Camera and optics manufactures apply lens coatings on the front element of most camera lenses in an effort to reduce fogging. In general, this works, but will NOT prevent fogging with radical temp changes.
You need to do what you can to attempt to minimize the changes in temp that the camera is going to undergo. I find it surprising that it fogged so severely. Was the housing in the sun prior to diving? Have you cleaned the coating off the lens? Did you blow on the lens prior to loading it into the case? Was the case stored closed where it could have built up moisture inside?
I would suggest keeping the camera/housing in a cold rinse bucket prior to diving to minimize the temp changes. Avoid radical temperature changes in general with gear. Make sure the housing is clean, and dry.
Was your lens fogged prior to loading the camera into the housing? If you do load a fogged camera into a housing, with no air flow, it will not generally get better.
I doubt a leak is a possibility as at depth that leak would have turned into a flood. My GUESS is that your camera was fogged at the surface and you did not realize it. Did you check it? Did you attempt to air it out between dives?
