Confusion about text box

I've been given a document to edit. It has two columns and many pages, all threaded together. The columns are fit to the margins and gutter. The text  usually flows normally, but in a few columns the text box is push down and there is a half, or on some instances, an entire line into the bottom margin. When I add text, it occassionaly does the same thing - push the text box down into the margin. 

I'm thoroughly confused. I thought that the text box restricts where text can appear, yet adding text seems to cause it to expand in some cases but not in others. What is going on here? How can I stop it?