Jenkins terminates due to an exception in the IRC code. I don't know how the exception is triggered or why, but I'm sure that a failure to echo something to an IRC channel isn't a terribly good reason to to shut down jenkins itself.

Here's the relevant log snippets:

Exception in thread "bot1-output" Aug 20, 2013 7:33:13 AM winstone.Logger logInternal
INFO: JVM is terminating. Shutting down Winstone
java.lang.RuntimeException: Exception encountered when writing to socket
at org.pircbotx.OutputThread.sendRawLineNow(OutputThread.java:70)
at org.pircbotx.OutputThread.run(OutputThread.java:104)
Caused by: java.net.SocketException: Socket closed
at java.net.SocketOutputStream.socketWrite(Unknown Source)
at java.net.SocketOutputStream.write(Unknown Source)
at sun.nio.cs.StreamEncoder.writeBytes(Unknown Source)
at sun.nio.cs.StreamEncoder.implFlushBuffer(Unknown Source)
at sun.nio.cs.StreamEncoder.implFlush(Unknown Source)
at sun.nio.cs.StreamEncoder.flush(Unknown Source)
at java.io.OutputStreamWriter.flush(Unknown Source)
at java.io.BufferedWriter.flush(Unknown Source)
at org.pircbotx.OutputThread.sendRawLineNow(OutputThread.java:66)
... 1 more