That's what the database is designed to do. It will consume available ram and use it for caching and other purposes. Once taken it won't let it go (because it assumes that if it needed in once it's going to need it again). It isn't a memory leak if you are worried about that.
You can set max and min memory use requirements. However I don't think this is actually a problem -- assuming that the only purpose those boxes have is to run sql server.
You can configure it according to the following documentation:
Msg 3903.. The ROLLBACK TRANSACTION request has no corresponding BEGIN TRANSACTION.http://nosqlsolution.blogspot.co.uk/