I believe the correct explanation is given by the answer of Rob Jeffries here :
The center of your wheels are traveling at the same speed as you. However the top of the wheel must be moving forward faster than this in order for the wheel to be rotating.
Thus it is possible that some of the muddy bits which get dislodged from the wheel, ( when the centrifugal overcomes the adhesion force,) leave off on a tangent and some of those bits catch up with your back as they leave tangentially with a higher velocity than the velocity of your back. Turbulence as described in John's answer will enhance the effect.
The rough argument is that the most backward part of the wheel is traveling with the velocity of the bicycle and your back. As the wheel turns if a bit of dirt dislodges due to loss of adherence it will have a larger tangential velocity than your motion with the bicycle and may hit your back. No laws are broken The spots in the picture are from muddy stuff. I agree that the water at three degrees will not turn and hit your back, unless with the turbulence mechanism described by John. Water itself does not have a high adherence and takes off before the angle is such that the tangential velocity hits your back.Mud has higher adherence.
Since the spots appear even though there is a narrow mud guard the tiny spots must be those bits of mud scattering off the circular edge of the mud guard some of it ending on your back, after all the spots are not so many, and mud breaks up easily so a tiny bit could hit your back.