It may be the anti-halation layer but most modern T-grain type films show this staining to some extent. Make sure your fix is fresh and the wash water is also warm enough as tap water is extremely cold at this time of year. Here's a bit from the Kodak site

With KODAK PROFESSIONAL T-MAX Films,
fixer will be exhausted more rapidly than with other films.
If negatives show a magenta (pink) stain after fixing, fixer
may be near exhaustion, or fixing time is too short. If the stain
is pronounced and irregular, refix the film in fresh fixer.