Too many stems diverts the plants energy into doing too much.
The same happens once blooming gets underway in earnest. The first blooms arrive atop strong sturdy stems, marvellous!
Then weeks go by and your stems seem to become progressively weaker, snapping under the strain??
What’s happening is the plant in its hurry to complete its life cycle is producing as many flowers as it can.
The heat we have been experiencing means a problem typical in Autumn is happening from the ‘get go’!