I had a similar problem getting a crank due to a previous "shop" putting Red Loctite on the threads. There was A LOT of cussing and cutting with that one. The reverse threads keep it from spinning off, no Loctite needed.
Do you think that the cage bolts were not tightened? Even then there should have been some warning noises or belts walking off?
What size hex bit is needed for the Lower?