Mine get that too sometimes. When deadheading the flowers, just trim the dead bits off. Are they receiving enough water? Or do you think it might be a garden pest?
Could be disease. Trim off all the dead stuff to prevent it from spreading.
Are they getting sunburned? Are they yellowing or browning or speckled?
That happened to my beloved geranium that I have had for years. I cut it way back, moved it to a sunnier spot, and watered it good. I also re-potted it, in case it was disease. I put more vermiculite in the soil to store water between waterings. It has come back in full this summer and you would never know it almost died. My family could not believe it was the same plant we had in early spring.