Ahh... my favourite svn tool svn blame
.
When you use TortoiseSVN, right click the file, select TortoiseSVN, select Blame...
Set your from
and to
revisions to narrow it down if you know approximate timeframe, else use 1
and HEAD
Do not select Use text viewer to view blames
While viewing the file with TortoiseBlame, hover over the revision column of the line you are interested in, and it will show the commit message text (with the revision number and author already shown
Right click on the revision column of the line you are interested in, and select Blame previous revision. Review the commit message and the author again. Repeat as much as needed to get to the bottom of this.