I know that this original post date runs back to 2007 but this seems to be a common issue still from what I hear around the place.
Apparently some guppies can carry disease (imports in particular), which they have developed an immunity to. When you add them to a tank full of guppies that have not been exposed to this before these guppies then die (not all of them, some like humans are strong enough to stand exposure). Do these guppies then become carriers? I don't know but I expect so.
I was told years ago not to mix batches of guppies purchased. When I did not mix them, there were no problems, when I did then there often were.
Fish coming out of quarantine still need to be isolated. Just because they have survived this process it doesn't mean that they haven't picked something up from the fish that died in quarantine.