Bluetooth audio sends data in small packets. If a packet does not arrive intact, the headphones cannot ask for it again the way a Wi-Fi device can. Instead, they either guess or drop the sound, which you hear as a skip or stutter.
Range and obstacles are the leading cause. Bluetooth is designed for a few metres with a clear line of sight. A body, a wall or even a pocket can weaken the signal enough to cause dropouts, especially at higher audio quality settings.
Interference from other radios matters too. Wi-Fi, microwave ovens, baby monitors and other Bluetooth devices share the same crowded frequencies. In a busy environment, packets collide and audio stutters even at close range.
The device itself may be overloaded. If the phone or computer is low on memory or running many tasks, it may not feed audio to the radio quickly enough, producing regular dropouts that look like a wireless problem.