Usually it requires heat to blow up a battery. If everything you did is according to normal battery care rituals, then I would have to say either shipping damage or MFG defect.

1. did you discharge and equalize the pack before charging
2. what was near, on in the vicinity of the pack before it blew
3. what was the charger settings at during the charge (amps, mv drop/cell, temp cut-off)

0.4 amps is very low rate to be charging batteries at, it serves no real purpose. You can accomplish almost the same at higher amp rates (4+), then trickle charging them to top off... I still see no need to trickle charge.