Why did I spin a main bearing?

On the upside- you didnt really spin a bearing. You have crap in the oil system definately... But you didnt spin it.
My take - that peice of junk may be material sheared off a cam bearing when it was installed. Did you use the right installation tool for them? The cam bearings look like crap and the cam never spun right. Bearing material could get through the pump, and is soft enough to imbed or stick to itself. If the block never went to the machine shop, there could have been some crap still in the oil galleys too. Basically, I think it was just "a matter of time", unfortunately. But, it didn't strand you, destroy the block, or really break anything important. You're only out the few parts you put in last time. Buy a book or two, follow what they say to do, and you should be fine.