Can that be because they were very well fed pets? I saw a piranha attack
on an adult water buffalo on TV. They took it down in minutes and in no
time at all it was reduced to a skeleton. :-(( I mean even the entrails
were gone!!!

These Piranha you saw are part of a tourism gimick in Brazil ( I believe
thats the place), somewhere Amazonian anyways. They dam off sections of the
river where they know there a schools of the fish and starve them for many
days. They then lead an unsuspecting victim to the water to drink. And
boom the rumors fly.

Sean