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Mike has the answer. It is the constant cycling of temperature which in time distorts the mating surfaces and causes the gasses/pressure/coolant to find the path of least resistance. This process is also very reliant on the quality and tension set on the fasteners and the order in which they are tightened, as well as the stages they are tightened in and the preparation of the mating surface .
Her is a link to my timing case bearing experiences. I reckon with some ingenuity you could possibly get the bearing out in situ even minus the center shaft, like a large bolt hooked behind the race. I don't think a puller with a center bolt that pushes in would be good as I fear this might push whatever is behind the bearing into the case (if it's like a welsh plug) or crack the case.
http://www.aulro.com/afvb/technical-...r-bearing.html
Dave
davros , its actually very easy to get the outer shells out , Simply weld on them and they will fall out , used that trick on much harder , blind hole shoulderless jobs than that one. Also freeze the new assembly in with a little dry ice . Wonder if you could make it "Greasable " by grinding slots down the edges of the shells and drilling and tapping in a grease nipple ?