Unexpected failover of RDS-MySQL server
Swapping is a bad symptom of over-use of RAM.
An excessive number of simultaneous connections may lead to swapping.
Very complex queries may lead to swapping (due to temp tables).  Let's see such queries if you have any.
Have you changed any things in my.cnf (or the equivalent in RDS)?  If so let's see if you raised some values too much.
As a quick fix (and if RDS lets you), decrase innodb_buffer_pool_size by a gigabyte.
Sort_merge_passes -  divide by Uptime.  1/second would be "high", but not high enough to be concerned with.
For deeper analysis, see http://mysql.rjweb.org/doc.php/mysql_analysis -- the GLOBAL STATUS and VARIABLES may find some clues; pt-query-digest will focus on the 'worst' queries.
sort_buffer_size = 2M is fine.
"ORDER BY clause not using indexes" - show us the queries, plus SHOW CREATE TABLE.