Your Sonicare toothbrush has Adaptive Intensity; a feature designed to help protect your gums from excess pressure.
If you brush too hard for an extended period of time, your toothbrush will automatically adjust the intensity setting.
When Adaptive Intensity starts working, you will notice a brief pause in the pressure feedback light and feel the vibration intensity adjust. Your normal settings will be restored the next time you brush.
Note: your toothbrush comes with this feature already activated. To disable it, see How do I turn the Adaptive Intensity feature on and off? within the FAQs.