The objective of this paper is to explain, in as much detail as possible, the physical mechanisms responsible for the reduction of skin friction in a microbubble-laden spatially developing turbulent boundary layer over a flat plate, for $Re_{\theta} = 1430$. Our DNS results with microbubble volume fraction ranging from $\phi_v = 0.001$ to 0.02 show that the presence of bubbles results in a local positive divergence of the fluid velocity, ${\bm \nabla} \,{\bm \cdot}\,{\bm U} > 0$, creating a positive mean velocity normal to (and away from) the wall which, in turn, reduces the mean streamwise velocity and displaces the quasi-streamwise longitudinal vortical structures away from the wall. This displacement has two main effects: (i) it increases the spanwise gaps between the wall streaks associated with the sweep events and reduces the streamwise velocity in these streaks, thus reducing the skin friction by up to 20.2% for $\phi_v = 0.02$; and (ii) it moves the location of peak Reynolds stress production away from the wall to a zone of a smaller transverse gradient of the mean streamwise velocity (i.e. smaller mean shear), thus reducing the production rate of turbulence kinetic energy and enstrophy.