What would cause a front tranny seal to blow?

How much fluid came out? I ask because when my buddys truck blew out the conv. seal it damn near puked its guts out in seconds. when we fired it up for just a couple seconds it coated the driveway. his ended up being a plugged up vent. I would suspect it may be something simple like Badart was saying and it just got hot and spit out the vent.