Why Do hot dog / Sausages Always Split Lengthwise?

Sausages universally split parallel to the length of the sausage. Why is that?

This behavior is well explained by Barlow's formula, 

The walls of a pipe (or a similar cylindric container, say, a sausage) experience two types of stresses: Tangential (σt) and axial (σa). For given pressure p, diameter d and wall-thickness s, the individual stresses can approximately be calculated from

σt=p⋅d2⋅s$$$$

and

σa=p⋅d4⋅s.

σ

Here, you can directly see that the tangential stress will always be larger, which is why it is likely that cracks in the container/sausage will first form in this direction. 

A real sausage will of course have various imperfections, which is why the crack path will not be straight in practice