They wouldn't have reduced memory usage without a lowmem signal from windows, so if SQL is reducing memory usage, there's something outside of SQL allocating and requesting memory and causing windows to issue low memory signals which in turn cause SQL to reduce its memory usage.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability