The curl is due to the fact that photographic film is comprised of multiple coats both emulsion side and base side. Each coat is slightly different as to its content so each has a slightly different rate of expansion and contraction.

The chief ingredient in most coats is gelatin. This is the flexible, transparent binder that glues the goodies onto the film base. When film is processed, the chemicals used are mostly water. Water wets the gelatin and it swells. This action allows the fluids of the process to enter and peculate about.

As the film dries, after processing, the gelatin shrinks back but not quite exactly back to its original size. This causes the curl you are experiencing. Films likely have a “balance coat” of the reverse. The balance coat ought to mitigate the curl. Not much you can do except press the film between the pages of a book. The curl is worsened under conditions of high humidity.

One remedy that might work if the curl is too wicked; at the drugstore procure some glycerin. Make a 25% of this diluting with distilled water. Soak the film for about 5 minutes. Follow with a solution made using Dawn disk washing detergent, two or three drops per cup. Hang wet film up and allow to air dry. No guarantee the curl will not come back!