When I flew from there 15 years ago, I was told that it had been built on landfill. This was borne out by the apparent shifting of the ground in the area of the south side tiedowns and hangers. The pavement was so distorted, while taxiing a Cessna 210, I was really quite concerned about a prop strike. The owner assured me it was fine as long as you taxied slowly, but it was still alarming.
A large hangar which housed a specticular warbird collection, had many cracks in the floor, and obvious shifting of the foundation. I have not been back since, I sure hope they got things fixed, though it must have been quite a task!
If you're flying around there, a flight up to Chino is well worth the effort if you are a warbird fan. Lots going on up there.
Pilot DAR