The reason is stated in
your ban lookup.
There were tunnels that led straight to mob spawners without any interruptions, nor second guesses. They were not close to each other either, so the excuse of "
I heard mobs nearby" is virtually non-existent. You have to be within 16 blocks of any mob spawner in order for mobs to spawn, which is pretty close-- none of the individual spawners were that close to each other. A normal player that would've heard mobs on a regular basis would've commonly found their way into a cavern, out of curiosity. This was not an act of curiosity.
Mob spawners are more rare than diamonds, and since they are a common ingredient for xp grinders, this would've been an easy inlet for you. However, since this had occured quite some time ago, I am willing to give you another chance. It is imperative that you re-read the server rules, as much has changed. Please be aware that we take mods and hacks seriously-- anything that grants a distinct and unfair advantage over everyone else is not acceptable, so it is best that you stay as Vanilla as possible. You may use Optifine if your machine runs slow when running Minecraft-- anything else is subject to the "
no mods rule."
If you can agree with the above statement, you will be pardoned. Anything less or not relevant will be ignored.