What would cause a front tranny seal to blow?

I had my transmission out last month and replaced the pump and also added 9 blue racing clutches.Now I,ve had this 904 worked on with lower first gear,changed out the drum,red racing racing belts,new pump and the 9 blue clutches.I,ve been racing with this tranny(904)and made about 30 passes behind the 408 in my 66 Dart.This weekend after about 5 passes,I made it too the topend of the track and noticed smoke.Well when I pulled over,there was plenty of tranny fluid leaking out.I,m assumeing it,s the front seal as the lines are all good.What would cause the seal to go so soon?Oh it,s a manual valve body also.Thanks in advance for any help.