Shattered flexplate!

I have had my 64' Dodge wagon back together and on the road this summer with no issues until this past weekend. I had the tranny rebuilt, bought a B&M stall, and a TCI auto/man valvebody. We went for a trip this past weekend with me driving my 31' and the wife following in the Dart. I woke up Saturday morning and found some tranny fluid under the Dart, not a lot, just a small puddle. While checking the fluid, I seen the trans was leaking while the car was running, again, not real bad just a drip, drip, drip. We headed home Sunday morning after I checked the fluid level. I did notice a slight rattle in park but the noise quit with the car in gear. We tried to baby our way home but about 50 miles down the road the wife called me and said she heard soemthing hit under the car and smelled something. I had her pull off at the next stop and jacked the car up. The smell she smelled was tranny fluid on the mufflers and the noise was part of the flexplate. When I looked there was 2 missing "ears" and big cracks in the other two! I can't believe the car was still moving! I am going to start to pull the tranny tonight and was wondering if there is anything I should check while it is out. This isn't a race car, just a mild street motor. I am a little curious to why the old stock flexplate let lose.?.