I've painstakingly frozen rows and columns in about 25 worksheets in a single
file only to discover a bit later that they've all become unfrozen. My sense
is that I'm probably doing something to cause this. Or is there some known
thing that causes frozen panes to unfreeze themselves?

A couple of possibly relevant facts:

-I'm keeping several workbooks open at the same time and I'm moving back and
forth between them, but I'm only freezing panes in one of them.
-I'm running several macros to put formulas in the worksheets, but there's
nothing that I can see in the macros that would affect the freezing.
-Worksheets are not protected

Any ideas?