Wyatt,
Your piston picture is a classic failure mode in air-cooled L-heads. Several things work together to produce such a condition. You'll note that the top land has suffered most in the area where the exhaust port is located in relation to the bore. Also note the severe coking of oil on the skirt in the non-thrust areas.
Here's the scenario...at temperature, due to location of exhaust port, expansion causes top of bore to distort and expand inward. This upsets sealing of the compression rings, allowing combustion gasses to leak by right at the highest pressure portion of the cycle. To add insult to injury, this is the major thrust side of the piston during the power stroke. The raking of the top land over a distorted bore, with a carbon-incrusted wear ring (potentially) protruding inward at the top of stroke, begins removing land material. Leaking of the compression rings (over oval bore) allows combustion gasses to torch past, further removing land material and forming carbon on piston skirt. Leakage results in elevated temperatures and a bootstrapping effect ensues.
So what you see is not a detonation induced failure, rather a bore distortion issue. It can be exacerbated by clogged cooling fins or stuck rings, as the other observers correctly contributed. The problem can be minimized by keeping cooling system clean and debris-free, running unleaded gas (lowering carbon formation), pulling head and checking for carbon (every 500 hours), and using MMO in gas. Synthetic oil will form less carbon in combustion chamber, and will also contribute to better lubrication of bore/piston interface.