Almost all speakers emit sound in 360 degrees though the exact ratio of the signal behind the speaker to in front of the speaker varies a great deal from speaker to speaker.
When you move a speaker closer or further away from a boundary (i.e. wall) these things change:
- Ratio of bass to mid and treble
- Time from first arrival signals to reflections from back wall
- Ratio of output of first arrival to reflected signals
So, that’s why. :)