Depends on whether they are indoors or outdoors. Indoors it is probable that they are in a too warm room and not getting enough water - only water from the bottom. Outdoors it could simply be the cold or lack of water. Of course as the flowers go over they do droop.

I agree with what Moon Grower says, or you may have Vine Weevil atacking the corm.

indoor ones that get left behind the curtains will do this too due to the colder air on a night.

Yup SBG :-)