Dynamic Crack Branching

Hi all, 

     I came across the problem of dynamic crack branching in few papers like ["Phase field approximation of dynamic brittle fracture", Schlüter et al (2014), "Phase-field modeling of ductile fracture",Ambati et al (2015)].  I am not able to understand the reason for the crack to branch out at an angle ~27 when a tensile uniformly distributed load is applied in a dynamic way at top and bottom side. If you take isotropic homogenous elastic medium why the crack should branch out like that ? I would be grateful if you could please share your thoughts on this. 

 Thanks in advance for your replies.