It seems to happen any time I put a footnote on a word of the final line of text. Not just once, but any time I have tried this it has happened. If it is the case that there is not space for the program to have the reference and the footnote text on the same page, I would think the proper response of the program would be to push that line of text from the final line of that page to the first page of the next. I'm surprised this is not an automatic response, so as to ensure that the two things occur on the same page.