1989 Jeep Wrangler Piston Broke

I have never had a piston just break. The engine started rapping bad like something just broke. I was thinking it might be a wrist pin. As I attempted to find the problkem I found the piston broke on the back side on number 3 piston. It is a fresh break the whole part that is broke. The cylinder doesn't look scared nor can I see anything the piston could have hit to make it break. I am thinking of putting a new piston in and putting the motor back together but I wonder why the piston broke and if it will just break again. It is a 2.5 liter. I pulled down three of the mian bearings and they show very little wearaand no scaring. The rod bearing for number three cylinder shows some wear but it didn't seize or turn. The driver of the vehicle (my brother in law) lugs the motor more than he races it. My question is I guess does anybody have an idea why the piston broke?