The value of owner field of a document is beeing changed when I copy a document from a library to another library which belongs to another site collection than the original library. I copy the document by opening libraries in Explorer view and copy paste the document. the value of owner field which is a custom site column is changed from domain\user1 to domain\members group.
This is because User field value uses IDs that are Site Collection specific. There is a hidden list at the root of each site collection where one row per user is created the first time the user is introduced in that Site Collection (e.g., user adds some content). List is located at /_catalogs/users/simple.aspx. List contains ID for each item (as with every list in SharePoint), which is a running integer from 1. SharePoint uses that ID when binding users to items within that Site Collection.
Apparently when a user with that ID isn't found in the destination Site Collection, SharePoint changes the person field in the copied item to something that exists for sure.
See this answer for few ways you can force sync the hidden User Information lists across site collections before you copy any items.