My MySQL Server Died
Honestly, I don't think mysqld is dying.
The message you posted reminds me of someone or something trying to restart mysqld in the absence of the PID file. When there is no PID file, doing service mysql stop does not work. I wrote about this in Percona-server time out on /etc/init.d/mysql start back on Dec 14, 2012 and MySQL Start/Stop 3 days later.
What's happening is that mysqld is already running and a mysqld start is being issued. This destroys the PID file (See MySQL Start/Stop under the heading PROCESSES (Shutdown)). The messages you are seeing is the second attempt to start mysqld only to find out that the plugins are already loaded.
What makes this more obvious is something I wrote back on April 24, 2014 in Two MySQL Server accessing same database over NFS:

Only one mysqld instance can attain an exclusive log buffer thread on a system tablespace
Only one mysqld instance can attain an exclusive insert buffer thread on a system tablespace

It only makes sense that the Buffer Pool cannot be allocated because a Buffer Pool already exists
SUGGESTION
Try shutting down mysqld with
mysqladmin -h127.0.0.1 -P3306 --protocol=tcp -uroot -p shutdown

This will prompt for password. After entering the password, if this returns no output, then mysqld performed a proper shutdown. (BTW this is handled in MySQL 5.7 because you can run the SHUTDOWN command from within a mysql client session). Then, you can run service mysql start with the PID file situation straightened out.
I hope you have the user root@127.0.0.1 or mysqld is smart enough to use root@localhost credentials (See my old post MySQL error: Access denied for user &#39;a&#39;@&#39;localhost&#39; (using password: YES) on how mysqld authenticates users)