Are the text frames part of a different flow?
You might wan to try a MIFwash (i.e. save the file as MIF, open the MIF and re-save as binary .fm) to clean out any crud that might have triggered it. See Re: Internal Error: 11024, 21101821, 21093447, 19354980 for details on using the Book MIFwash utility in FM11.
This has happened,
heck still happens to me.
(more than once)
even today, to ONE chapter of a 14+chapter document..
I too did not CHANGE any of my settings, it just decided it wanted my text boxes to have side headers.
(which is why I found your question while trying to find an answer.)
The fastest way I have found SO FAR to fix it, is to
Select each text box,
The Text Frame Properties box shows up, and it has a check box in the Flow -Room for Side Heads pane.
Now, I have no idea why, but my FM11 decided that it wanted 1.5" with .25gap on my text boxes.
I had to un-check, and Apply.
but it was easier than copying the information and putting it in a new text box, (which oddly enough, also works... )