Why did I spin a main bearing?

not sure. i havent taken it apart yet. i just cut apart the filter and some bearing chunks found their way in their in there. I guess im not sure its a main bearing but its definately in the bottom end. also last year i ran valvoline race oil 20w 50 (high zinc) and made the switch to amsoil this year. could i have a compatability issue? it ran 500 miles this year