Could be humidity.
Emulsion swells and shrinks at a different rate than paper as it absorbs or releases moisture.
If emulsion gets drier than than the paper it will shrink, causing the paper to curl toward the emulsion side. (Positive curl.)
If emulsion gets more moisture than the paper, it will expand, causing the paper to curl the other way. (Negative curl.)
To a certain degree, this is normal. Film or paper will always curl, depending on the humidity of its surroundings. If the film curls too much or if you try to force it flat when it doesn't want to, you can damage the emulsion. However, most of the time it doesn't hurt.
Just regulate the humidity of the surroundings where you store your film, negatives or paper. Try to keep it around 50% relative humidity. Limits should be within 40% and 60% with 50% being the average. Over time, the paper should lie nearly flat if it is in the right humidity.
If the case is extreme, you could put the film/paper in a humidity chamber.
Get a plastic container that seals tight. Place a wet sponge in a shallow dish of water. Put your film inside the container, being sure the water can't make direct contact with the film, and seal it up for a day or so. The humid atmosphere in the container will add moisture to the film and allow it to flatten. If the film is too moist, you can do the same thing except use silica gel crystals to absorb excess moisture instead of a wet sponge.