Why did I spin a main bearing?

I had .002 clearance plastigauged on all mains and crank was polished. Engine ran great for 3500 miles and with no warning at all I lost oil pressure and it died immediately. Nothing wrong with oil pump, drive shaft, cam gear or timing chain. running amsoil z-rod 10w 30 @ 60psi when its warmed up at idle. What did I do wrong?