Why did I spin a main bearing?

it didnt lock up, just tight. I did not check the main caps, i thought since it was a good running engine a quick re-ring and bearing change would be all it needed. i was on a really tight budget but looks like i didnt save myself any money now that this happened. I primed it before it was started band i did change all engine bearings when i did the quick rebuild and cam change. cam bearings were a little tight when they were installed but my "mentor" assured me they would wear in fine. since i have the timing chain off i pulled the rocker shafts and spun the cam and it spins nice and freely. Ill tear it down tomorrow and post some pics of what i find. Thanks for all your input. If nothing else, hopefully someone can learn from whatever it is i did wrong