.bak files are usually backup files created by a program when needed to modify the original.
I don’t think they are anything to do with Traktor - probably a different program - you should be able to delete them and still open the original. Try one.
edit:
Traktor probably does create a backup file when analysing / writing tags - if so it should delete it after it’s modified the original, so may be a Traktor problem, but I doubt it.
Thanks pal, i think that it could happen because my collection has broken a few times and i didn’t let traktor analyze all files before closing it (because i have 60gb of music), and so the files couldn’t be deleted…