More silverfish appearing? How do I stop this?

No, this is not a bug. It is an intentional feature. (though I've never seen it in Hunger Games before)

In Minecraft, there are "bug blocks" accessible in the creative inventory. They look exactly like stone bricks (or a variation of), stone, or cobblestone. These blocks are called ID's similar to "Stone Brick Monster Egg."

Silverfish have a special property - when they get hit, they summon more. They can only summon more from these bug blocks, destroying the block in the process*. If left alone, a silverfish will disappear into a non-bug stone, cobble or stone brick block and turn it into a bug block.

Whenever devious map makers want to cause chaos, they often spawn in a silverfish and surround it with bug blocks. Apparently the Hunger Games map had bug blocks, and you attacking the silverfish summoned more.

*Bug blocks also summon silverfish when broken by the player.

You can prevent a horde of silverfish by:

• One hitting the silverfish. Silverfish which are one-hitted (killed in one strike with a weapon) don't summon more, even if there are bug blocks around.

• Not digging bug blocks. When you try to dig a bug block, it will dig sort of slow compared to a pickaxe, but quite quickly compared to the fist. It is a unique digging speed, meaning it can't be reproduced with a pickaxe - so experienced players can tell if they are digging a bug block.

• Running. Though it might be obvious, as with most any other slower-than-player mob your best chance of survival is running.

There is a really easy way to get rid of a swarm of silverfish in Creative Mode: dig down until you fall out of the world and, when you respawn, all the silverfish will be gone!