The private pilot landed his single-engine tricycle gear homebuilt airplane at a private 2,600-foot long by 200-foot wide grass airstrip. The pilot reported, on approach the airplane had about a 25-degrees of flap setting; he then "slowed [the airplane] to 80 knots on final [and crossed] over the fence at 70-75 knots." The pilot added, he made a normal landing and held the aircraft's nose wheel off the surface of the runway, until the airspeed bled off. About halfway down the runway, he stated that the airplane's nose suddenly lifted to about a 25-30 degree angle and when the nose came back down, he felt a "shutter". After the nose wheel came in contact with the runway surface, the aircraft tipped forward and while at a very slow speed, flipped over. The aircraft came to rest in the inverted position. The pilot inspected the runway and reported that about 66-feet from the aircraft, it appeared the nose wheel contacted the side of a "gopher mound" which caused the aircraft's nose to elevate. About 36-feet from the mound, and about 6-feet from where the nose wheel contacted the surface, the nose wheel track showed signs of widening. The wheel track got progressively wider and deeper until the point where the aircraft overturns. The aircraft had not been modified with the kit manufacturer's service bulletin, which increased the nose fork axle-to-ground clearance; the pilot added that he didn't think the added clearance would have helped prevent the accident.