A CPU Usage alert is a Threshold Alert that notifies you by email when the CPU of your Solr deployment exceeds a specified threshold for a period of some minutes. This creates an Incident in the Managed Solr Dashboard.
CPU Usage alerts can result when the system is under heavy stress.
See Is 100% CPU a Bad Thing? for a discussion of how an overloaded CPU can cause cascading failures that can bring down a cluster.
Overloading is often associated with inappropriate indexing configurations. Consider adjusting your commit strategy to place less load on the system.
This situation can be relieved (not necessarily cured) by performing a rolling restart to clear the queues. Contact SearchStax Support if you need any help there.