Why is Latex hyphenating some words automatically, but others dont?

At first the word \LaTeX right before usando is confusing the hyphenation. But most importantly: \righthyphenmin is 3 in this language, that means that LaTeX will never break 2 chars at the end. You can change this:

\documentclass[10pt,openright,twoside,a5paper]{memoir}
\usepackage[brazil]{babel}

\usepackage[showframe]{geometry}
\setlength{\parindent}{1.6cm}

\renewcommand\brazilhyphenmins{22} %left=2, right=2

\begin{document}

\frenchspacing
\selectlanguage{brazil}
No entanto, você pode compilar os documentos \LaTeX{} usando os seguintes
comandos, que devem ser digitados no \emph{Prompt de Comandos} do Windows ou no
\emph{Terminal} do Mac ou do Linux:

\end{document}

Ulrike's answer is right, of course, but such problems can often be prevented by loading the microtype package. Here that gets

\documentclass[10pt,openright,twoside,a5paper]{memoir}
\usepackage[brazil]{babel}

\usepackage[showframe]{geometry}
\usepackage{microtype}
\setlength{\parindent}{1.6cm}


\begin{document}

\frenchspacing
\selectlanguage{brazil}
No entanto, você pode compilar os documentos \LaTeX{}  usando os seguintes
comandos, que devem ser digitados no \emph{Prompt de Comandos} do Windows ou no
\emph{Terminal} do Mac ou do Linux:

\end{document}

EDIT

What microtype does is play with all sorts of typographical features to make words fit better on a line and to make the end of the line look more even. It's clear that it helps here.