What would be the cause..................

[Fay]

............ a gray 16bit tiff would change to a RGB 8 bit?

[Mac]

assuming your on PS.
probably your settings,
did you save it as a 16bit RGB or Grey, or as an 8 bit
have you got the setting set so that it opens 16 bit as 8 bit rgb?

check your camera RAW settings in PS as that is where the settings for opening tiffs are.

Mac.