Before revision:  The driving mechanism for this potential singularity is due to two antisymmetric vortex dipoles that generate a strong shearing layer in both the radial and axial velocity fields , which transport the solution first towards z=0 and then towards the symmetry axis r=0.
After revision:  The driving mechanism for this potential singularity is due to two antisymmetric vortex dipoles that generate a strong shearing layer in both the radial and axial velocity fields . Without the viscous regularization, the 3D Euler equations develop an additional small scale characterizing the thickness of the sharp front. On the other hand, the Navier-Stokes equations with a constant diffusion coefficient regularize the two-scale solution structure and do not develop a finite time singularity for the same initial data.