mysql no longer defaulting to /tmp/mysql.sock
The MySQL documentation states that the default path to mysql.sock is /var/lib/mysql/ on RPM distributions. The path is set at compile-time depending on the target OS and can be overridden by configuration or command line, so for CentOS the behavior you are seeing is correct.
This means that the socket path must have originally been set to /tmp by some other mechanism. There are several places this could have been done; likely in the init script /etc/init.d/mysql.server (to set socket=/tmp/mysql.sock on the command line - this can be done in an init script even though it is unsupported) or a local options file. See this page in the documentation for MySQL's precedence order when processing startup options. Entries listed later in the table will override the preceding ones.
It probably isn't possible to trace the sequence of events leading up to the socket path change, but fortunately you can change it via ~/.my.cnf, the user-specific option file.