Shipping damage...what would cause capacitors to leak?

I sent a NuPrime ST-10 via UPS Ground from San Diego to the San Francisco Bay Area and the buyer found that there was black goo leaking out from the back of the chassis.  Upon opening the amplifier he found that two of the capacitors were leaking.  Being of a non-technical nature, I'm wondering what might cause something like this to happen?  Could it be from impact?  Or maybe the package was left in a truck in high temperatures?  I sent just before July 4, so the package was probably sitting outside in a truck for the holiday.