Wow, that really sucks. The only things off-hand that I can think of that would cause piston damage like that would be either preignition (aka spark knock, burned a hole in the piston big enough to cause it to disintegrate) or over-revving. I've never really seen a piston give due to over rev though, usually its a rod that lets go. I wouldn't rule it out. The only other thing I could think of would be too close of tolerances in the cylinder. Maybe it got just hot enough so that where there would normally be enough clearance, it expanded to the point where the piston contacted the cylinder wall, got hotter, expanded more, seized against the wall, and then the inertia of the crank pushed the rod through the piston...wow. thats a lot of "what-if" and "perhaps". Make sure that the shop checks the rod and crank. Best bet would be to replace the rod no matter what. You might also want to have the block and crank magnafluxed. Its when they check for small cracks and weaknesses that visual inspection can't catch. Either way, here's a good excuse to put in that 440 or 465 kit. Good luck, sorry bout your motor!
2003 TRX400EX
Mods: Yamahonda conversion (Thanks Corry), Yoshimura Comp exhaust, Hot Cam stage 1, Wiseco 11.2:1 piston (stock bore), K&N Airfilter w/ outerwear, 158 main jet, 42 Pilot, #3 Groove on the needle. Faster than I expected.
1996 Blaster
So far: Vito's Piston/ Jet Kit, 28mm PWK Carb, Ported cylinder, K&N Airfilter, Hydraulic rear brakes, AC Racing front bumper and Nerfs, Graydon Proline pipe/silencer, Silver PC frame, Maier Race plastics, FLY handlebars, Banshee Shocks. Faster than you'd expect.