If the open lid on a mature pitcher closes up. The main cause is a drop in humidity beyond what the plant is able to handle with moisture uptake. Excessive heat can lower the plants ability to deal with humidity drop also as the plant is naturally transpiring faster.
The other causes are related to the plants root structure and health. The larger and healthier the plants root system the better able it is to absorb moisture and keep up with humidity fluctuation. Of course the other possibility is that the roots are too dry and there simply isn't moisture for the plant to access.
Do these pitchers lids open back up? So that they are closing during the day and then overnight they open?