Why is a glottal stop often heard in English words such as “bitten” and “fatten”?

In English, the use of a glottal stop in words like "bitten" and "fatten" is a common variation in pronunciation related to the timing and state of the glottis during speech.1

A glottal stop is a momentary check on the airstream caused by closing the glottis—the space between the vocal cords—which stops their vibration. While the glottal stop is not a distinct phoneme in English, it functions as an allophone of the t phoneme in many dialects. In words such as "bitten" and "fatten," the glottal stop occurs as a result of the vocal cords being held tightly together, creating a brief, coughlike explosive sound upon release. This process, known as glottalization, involves the partial or complete closure of the glottis, which can occur before, during, or after the primary articulation of the sound.21