We can't tell what's caused the stems to flop or break by viewing the photos.  But we know it's disappointing to see what's happened when the plants are lovely and in full bloom.

Here are some possible explanations if someone/or something didn't step on the plants:

Stems may be weakened by disease, especially near the crown, making them brittle and easily broken.

Giant flowers, while lovely, might make the plants top heavy--too much for the stems to support.  If that's the case, cutting some blossoms and/or removing any that are spent might reduce the problem.

Too much water or shade can produce tall weak stems. 

Heavy rain and wind could be factors.