Not a doubt in my mind, zero, that the trans removal and install was the cause of a failed flex-plate. If the trans was forced on to a torque converter left bolted to the flex-plate, bad juju happens! If the torque converter was on the trans input shaft and bolted to the flex-plate incorrectly during install, bad juju happens! If the flex-plate wasn't inspected before the trans install, bad juju happens! As a matter of course, the flex-plate should ALWAYS be replaced in a high-mileage trans install, or BAD JUJU HAPPENS!!