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



This behaviour is well explained by Barlow's formula, even though the English Wikipedia article is incomplete in this context. The German version, on the other hand, gives the full picture (which I will quote in the following).



The walls of a pipe (or a similar cylindric container, say, a sausage) experience two types of stresses: Tangential ($\sigma_{\rm{t}}$) and axial ($\sigma_{\rm{a}}$). For given pressure $p$, diameter $d$ and wall-thickness $s$, the individual stresses can approximately be calculated from
$$\sigma_{\rm{t}} = \frac { p \cdot d } { 2 \cdot s }$$
and
$$\sigma_{\rm{a}} = \frac { p \cdot d } { 4 \cdot s }.$$
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. In fact, this is why the first formula is often stated on its own, just as it is the case in the English Wikipedia article.

Fun fact: The sausage example is used by many German students as a mnemonic helping to remember which of the stresses is larger. As a result, the formulas are often called "Bockwurstformeln" (sausage formulas).

Edit: In response to the comments below, I will try to summarize some details about the above formulas


The formulas do not directly indicate how and where the container will split. Assuming that the tensile strength is identical in all directions, we can see that there will be a greater release of tension when the crack propagates length-wise (See the video posted by JoeHobbit and the comment by LDC3)
A real sausage will of course have various imperfections, which is why the crack path will not be straight in practice

