In this document, where I am replicating my original document in order to get the issue I am highlighting here, a comma is stranded on the next line:
\documentclass[a4paper, 14pt]{extarticle}
\usepackage[paper = a4paper, margin = .5in]{geometry}
\usepackage{fontspec}
\setmainfont{Linux Libertine O}
\usepackage{microtype}
\usepackage{gb4e}
\exewidth{(23)}
\begin{document}
\begin{exe}
\ex The suffix \textit{-ung-}, on the other hand, clearly undergoes vowel harmony here, cf.\ \textit{áttong-}, \textit{brǿðrongr}, \textit{fjórðong-}, and \textit{konong-} vs.\ \textit{systrungr} and \textit{þriðung-}. These words occur frequently and there are no exceptions.\\
\end{exe}
\end{document}

Why does this occur and what is the best way to fix/prevent it?
There's a similar problem raised in How to avoid a linebreak before a comma?, but the accepted answer refers to the babel package, which I am not using (I use polyglossia), and to its documentation, whose current version doesn't mention the "~ sequence given in the answer.

babelshorthands=on. -> From the package:\DeclareOption{babelshorthands}{\setkeys{polyglossia}{babelshorthands=true}}– Johannes_B Mar 17 '15 at 16:02\mbox{-}where you wanted to avoid mis-hyphenation. – Steven B. Segletes Mar 17 '15 at 16:17\nobreakdashdefined inamsmathfor this express purpose. if the description of the problem in the user guide (texdoc amsmath, p.9) sounds like it would do what you need, i'll be happy to define an analog that shouldn't step on the toes of the class file and packages shown in your example. (it would not be automatic.) – barbara beeton Mar 17 '15 at 16:17textitall the time. – Johannes_B Mar 17 '15 at 16:18mboxis the way to go, and who am I to disagree?:)– Sverre Mar 17 '15 at 16:47\textitworse than repeating some other command I define as equivalent to\textit? – Sverre Mar 17 '15 at 16:48\refstepcounter{section}\large\bfseries\thesection~this is my section title\par\normalfont\normalsize\noindentover and over again. Or whatever the definition of section in detail is. – Johannes_B Mar 17 '15 at 16:50affixto useslshapeisntead ofitshape. Something else. – Johannes_B Mar 17 '15 at 16:51:)(Since the length of\textitis equivalent to the length of some other equivalent command). – Sverre Mar 17 '15 at 16:54memoirand theKOMAclasses provide hooks and stuff to definesectionfontand similar stuff. – Johannes_B Mar 17 '15 at 16:56hyphencomma.styfrom http://tex.stackexchange.com/questions/268794/how-to-avoid-a-linebreak-before-a-comma-in-lualatex works fine with your example in LuaLaTeX – georgd Sep 23 '15 at 07:03