Tasks are being created with a "Related to: Accounts" field. 

At some point this Related to field is being cleared. 

What we are left with is a number of Tasks that now do not have an associated Account. 

We have explored this being an issue with user input but as a task cannot be created without a related to field we have begun to suspect this is a problem behind the scenes. We have not been able to replicate the problem but slowly our list of Tasks with no associated Account has been growing.