I think this because they are not line dried after use and this makes the moisture to accumulate causing the growth of mildew. You can eliminate such smell by washing the towels using vinegar and then rinse them well and line dry them after removing from the drier.
If at all possible, line dry towels. They will smell like fresh air and sunshine instead of mildew or stale air from dryer. The only time I use my dryer is if it is raining and I can't hang my clothes and other items outside.