My best guess would be that the pansies are too wet. Perhaps there is a drainage issue of some sort or you are overwatering. Pansies like the cold so I don't think it's that, but cold weather often comes after a front with rain, so I think it's the water.
Another cause could be some kind of wilt - bacterial or viral or something - I'm not an expert here. But maybe consult with a good garden center/nurseryman for some kind of treatment like a soil drench that you could do. I would first try cutting back on the watering or check the drainage in the containers. Are the containers very large? Could they be put up on a stand or pot feet?