How did || come to be used in crypto texts to represent concatenation?
The origin is set theory and not programming languages.  In the context of cryptography, I could describe a set that is 
$$x_1 \parallel x_2 \parallel \dots \parallel x_n$$
as a concatenation of the series described by
$$\parallel_{i=1}^n x_i.$$
Furthermore, it's worth noting that + to a mathematician would suggest that it is a commutative, which might not be true depending on the set (as we could have a set of functions).