According to the wiki on forced, it's for "one character making another character doing something they do not want to do". However, with forced_transformation there isn't necessarily another character causing the transformation.
Sometimes its a cursed object:
post #1967951 post #2231850 post #1876486
Sometimes a character that may or may not have caused it is nowhere to be seen:
post #2142034 post #1493355 post #1905356
And of course the good ol' full moon:
post #2107475
These examples shouldn't have the forced tag with its current definition, but are stuck with it because of being a "forced"/unwilling transformation. As an alternative, the definition for forced could be changed to not rely on a secondary character doing the forcing.