The process of sound propagation can be a bit counterintuitive at times. When you speak, sound waves are generated as your vocal cords vibrate. These sound waves travel in all directions away from your mouth, including towards the front, sides, and also towards the back.
Even though compressions and rarefactions are formed in front of you due to the sound waves propagating outward, some of these sound waves also wrap around to the back and sides of your head. These sound waves can then reach the ears of someone standing behind you, allowing them to hear your voice.
Several factors contribute to this phenomenon, including diffraction and reflection of sound waves. Diffraction is the bending of sound waves around obstacles or openings (such as your head), and reflection occurs when sound waves bounce off surfaces and change direction.
Furthermore, the human body, including the head, is not a perfect barrier to sound. Some sound waves can pass through and around the body, which is why someone standing behind you can still hear your voice, though it might be somewhat attenuated compared to someone in front of you.
To illustrate this, you can try an experiment: stand with your back to a wall and have someone speak to you from behind. You may notice that you can still hear their voice, although it may not be as clear as when they are facing you directly.
In everyday situations, this phenomenon isn't usually a problem, but in environments where privacy is essential, such as during a private conversation, it's best to be mindful of the potential for sound waves to propagate in various directions and take steps to ensure privacy.