Most probably a narrow column with a word near the end that TeX had problems hyphenating.
A line of text is basically a hbox. The words in this line are fixed in their lenght, so TeX distributes the space between them as evenly as possible to fill this hbox. It has a certain range for the length of a space, and tries to move words or parts of words with hyphenation around to stay in the OK range for the space width. If it can’t, it complains about under- or overfull hboxes.