The thread pool for AlwaysOn Availability Groups was unable to start a new worker thread because there are not enough available worker threads

 In the event of excessive worker thread contention and long term failures to start a worker, after 15 minutes there will be a logged message in the error log.  It is message id 35217, and in your error log the message will read:

“The thread pool for AlwaysOn Availability Groups was unable to start a new worker thread because there are not enough available worker threads. This may degrade AlwaysOn Availability Groups performance.  Use the "max worker threads" configuration option to increase number of allowable threads.”

Follow this link :- 

https://blogs.msdn.microsoft.com/sql_pfe_blog/2013/07/15/monitoring-sql-server-2012-alwayson-availability-groups-worker-thread-consumption/

take a decision to increase CPU.

Thanks for reading..