This alarm is raised when the Tempdb Version Store growth rate is high compared to the cleanup rate.
There are two version stores in Tempdb. The online index build version store is used for online index builds in all databases. The common version store is used for all other data modification operations in all databases.
When the alarm is raised, investigate the Amazon RDS for SQL Server | Tempdb drilldown | Usage page | Tempdb Version Store Generation Rate chart.