Why did I spin a main bearing?

What does the cam look like? Are the lobes and lifters wiped? I don,t see how a piece that size would make it through the screen in pick up. Looks like trash that was in oil passage somewhere. I would also look at oil pump shaft as it may have broke off. Before you put it back together I would knock out the oil plugs and get long bottle brush and scrub block real good with soap and water. Blow it dry with air the spray it with rust inhibitor.