According to the BMW SIB that is linked on the first page, excessive heat during manufacturing, at the factory, caused the torque of the bolts to change due to something called 'creep' where metal stretches / moves when it's heated. As far as I understand it, the continued high temperature of the exhaust side of the engine during the car's life causes this creep to continue, which is what has happened in flimper's pic. So I guess that a bolt falls out, others loosen and the weight of the engine and the torque (movement under load) the remaining bolts experience eventually leads to shearing of them. It is NOT the bolts top and bottom of the mounts themselves that fail, it's the bolts holding the plate those mounts sit on that are attached to the chassis and block that fail.
It's a known problem, and some people have successfully claimed recompense for repairs from BMW... I'm not sure what I will do, depend on the cost / hassle of doing so. No warranty, so I can't go that route, probably not worth it.
I haven't had an update today, so I'll give the garage a call tomorrow.
EDIT - can't find the link to the SIB, the ship killer link isn't working and I can't remember where I clicked on it. Will keep trying
EDIT #2 - here's the SIB : https://www.bimmerfest.com/forums/attachment.php?attachmentid=290053&d=1314055257
It's not the same one as I've seen before though, there were pictures in the other one...
Final edit - it's this link from above, has the SIB, info on creep, pics of the mounting plates : http://www.bimmerfest.com/forums/showpost.php?p=3887169&postcount=39