800 GS Back bearing probs ?

Recently got back from a solo trip to Zanzibar and back, both back bearings went on different occasions ( I fixed the 1st one and then about 1000km later the other went, which wore spacers etc and ultimately ended my trip ) ... My bike has done about 33000 km, just a few questions ... Why would both bearings have gone in a 1000 km space, is it just their time ? Was it the weight on the back bearings, about 30kg, was it the terrain ( i mainly did tar with some really muddy, wet dirt sections ), Could it have been a bike shop ( i had new tyres fitted just before i left, rode about 2500 km on them before 1st bearing went ), was my shock set to hard ( i only increased it by 1 turn because of the weight ) ?

Let me know if anyone has had similar problems or what you guys think ...

Thanks