If you have 1 broken piston, and all of the parts that you put in that motor were the same, (I'm assuming that you did use a set of matched pistons..
), then you need to pull the heads and pan to see why. At the very least, you need to see if you smashed the upper ring groove.
It's just that simple.
Maybe you dropped a bolt or something down the intake, and that's what caused your mishap. ( I just tore one down last month for a guy who had too many people around his motor while his car was being built....stainless washer dropped down the intake, beat up the head and piston)
Maybe you put a piston in backwards.
Maybe you have some severely mismatched parts.
What kind of pistons did you put in that motor?
Are they high compression?
What are the camshaft specs?
If you have a high lift cam, and high compression pistons, did you check the valve to piston clearance with clay, like you were supposed to with a high lift cam and high compression pistons?
You will only be guessing until you start looking at the actual clearances that are present in your build.
You don't know that a rod is not bent until you actually look at it....You can not know if a ring land is not smashed now until you pull it apart.
Who selected the parts? Who did the machine work and fitted the pistons to the rods?
You are asking what could have caused this problem, but you have given us very little info to try to help you.
You say YOU rebuilt this motor, give us ALL the specs of what you have installed..