SQL Server 2014 Buffer Pool Extension - There is insufficient memory available in the buffer pool
The best answer I can come up with is that Sql really doesn't have any room in the buffer pool to give up.  Meaning, we have too many queries running, or the queries running have plans that are taking up too much space.  This link was somewhat helpful No Free Buffers.  My solution at this point is to evaluate the queries that were running when the error occurs, so see if I can tune them.