The first thing to make the flowers bend, apart from wind, rain, and physical factors, is generations of plant breeders growing them to be big blousey plants for the market and consequently the plants can be top heavy if not staked.
IMO.
Apparently, it is a common problem caused by the weight of the flowers, once they have bloomed, which might make the stems tip to the side or even break.
This link offers some suggestions as to minimise the problem:-