swing bearing bolts breaking

What causes the swing bearing bolts to break and snap off at the threads? Just got it into the shop and giving it a once over and fixing things and noticed some bolts missing out of the swing gear housing underneath, after some investigating, they snapped off at the threads, the machine has about nine thousand hours on it and I'd guess its time for the bearing assembly as well, but how do you get the broken bolts out, the upper structure off and why did the bolts snap off in the first...

swing bearing bolts breaking

swing bearing bolts breaking