Just stream-of-consciousness spit-balling here, not an essay.
Get crossed up in a crosswind, panic and stand on one rudder + brake. Have the nose gear wheel fall off & the strut digs in.
Agree that SMO is pretty much one big rectangle of asphalt/concrete these days, but there’s probably some big seams or drainage ruts or something that if you get crossways enough to the runway you dig in something and flip.
But with no crunched wingtips or stab tips, all 3 gear intact, and the prop still attached it’s a mystery.
One of my Dad’s C-150s ended up on its back after the not so bright PP taxied right up behind the left engine of an Air Cal 737-200. Who then gassed it to cross the threshold of one runway on the way to the other. Instant flip.
No 737s at SMO, but if he got too close to a large enough helicopter lifting off plus some wind that might do it.
Looking at the lead pic in your cite, there is a suspicious gouge / skid mark coming up from the bottom of the pic and ending near the nose of the aircraft. If that was from this airplane, his path was about 80 degrees off from the runway alignment.
Looking more closely at the pic in high zoom, I think maybe his left gear leg is damaged. Sorta like the axle assembly came partly off the leg or the leg snapped just above the axle. The wheel & tire are still attached but the spatial relationship between the leg & wheel/tire look very different from the right one.
If so … he lands, unaware anything is wrong. Either the gear fails on touchdown or shortly after. Now he’s skidding on the stump of steel on the left side with normal tire & brake on the right side. Real quickly he’s going to be veering off the left side of the runway. The gear “trips” over a small obstacle, perhaps even a runway light, and he’s flipped.
All guesswork, but not insane.