I doubt that anyone can tell you unless you provide more specifics. Under
what circumstances does it change? From what to what?

By default, Word 2007 Headings are set to use Cambria, and body text is set
to use Calibri. So, if you're using the Heading 1, Heading 2, etc. styles
for headings, and Normal for body text, that would explain at least in part
why they're changing.

More specifics--as Suzanne indicates--would let us zoom in on what's
happening in your situation, as well as how to change it if it's a problem
for you.

Cheers,