The official website speculates that extraterrestrials buried unearthly metals or a spacecraft beneath the Spot, or that carbon dioxide seeps out of the earth. Another popular speculation is that gravity is affected here, causing nausea and incorrect instrument readings.
And the Illusion explanation is like this:
One of the many optical illusions at the Santa Cruz Mystery Spot.The Mystery Spot is a tilt-induced visual illusion. The illusion experienced by visitors results from the oddly tilted environment as well as standing on a tilted floor. Inside the tilted room of the Mystery Spot, mis perceptions of the height and orientation of objects occur. Even when people are standing outside on a level ground, the slant of the building in the background causes mis perceptions as we judge the height of people using the slant of the roof rather than the true horizon.