The gearbox on my tenere failed at just 27k...

Without any other previous symptoms, I started hearing some rattle while waiting at a red light while the transmision was in neutral. It stopped and rode home without any issues. Next morning I started the engine and while putting the helmet i heard the rattling again... on the road to the dealer it felt like at certain times the transmision was about to slip out of gear while riding constantly at 3k rpm... ironically when i was trying to show the mechanic what the symptoms are it all went smooth for about 20 mins - finally, it started again but this time it was on centerstand so i could see better what happens: bike out of gear, at around 3k rpm it seems like it couples first gear for a fraction of second, rattles and moves the sprocket few degrees...

Bike is left in the shop to open up the gearbox...

I haven't abused the bike much, changed the oil at every 5k and it seems to me a bit earlier for the gearbox to fail at just 27k under these circumstances... any idea for what might cause it?