In case you haven't figured out exactly what that means, here is an example:
A fireworks stand gets set on fire. Bad news #1.
Across the street, a tanker truck full of gasoline spills over. Bad news #2
It is July 4, and the street in between them has a parade going through it at the moment. Normally not bad at all.
It is a very dry, windy day. There hasn't been rain for a few weeks -- the grass is mostly dry. Moderately bad news.
There is a fire on the other side of town, and 3 of the town's 5 fire trucks are there fighting the fire. 6 of the town's 20 firefighters are down with the flu. Moderately bad news.
Now all of those things could become a "perfect storm" where the fireworks shooting off in all directions INTERACTS WITH the gas fumes in the air, which INTERACTS WITH the dry grass all around, and any explosions and fires affect the people watching the parade. All of the above becomes even WORSE because the city can't mount an effective battle against the fire, since the 3 firetrucks must finish up the fire they're fighting, then drive across town. There are fewer firefighters to draw on, as well.
Any one of these things would be bad news, but TAKEN TOGETHER they have a multiplicative effect -- much greater than the sum of the parts.