Why does svn diff sometimes copy working files to a temp file?

Do you have any svn:keywords set on those files? If yes, then Subversion will create a temp file first which has all those expanded keywords 'unexpanded'. This is to avoid having all the keyword lines shown as different even if they haven't been changed.

For example, if you diff a file in your working copy against BASE, the BASE file has no keywords expanded (and is also stored with LF lineendings), while the file in your working copy has all keywords expanded (and may be stored with CRLF lineendings). If you haven't modified that file locally, 'svn diff' would show all lines as different (if the line endings don't match) or at least all lines with the keywords would be shown as different - which is not what you would expect.