Engine self destruct

New to this forum so i thought i would share what happened to me yesterday when riding my scooter…left hand turn…down to 2nd from 3rd…accelerate out of corner…bang…crack…grind and the scooter locks up so in with the clutch and pull over to take a look…nothing shows up so i make the short journey home…pull the motor to bits and find that the primary gear set has exploded and punched a hole in the crankcase and taken 2nd gear with it…any ideas anyone…scoot is only 18mth old…spec is listed below and i have attached a couple of pics .

166 malossi
cases matched
20 mm removed from crank
26 mm dellorto (120 main)
scorpion pipe
lightened flywheel
k/n filter

scoot has been fine for 8 months?