My Mom has been diagnosed with dementia and is now spitting on the floor. Why?

Why? Because she has dementia. Remember, when someone has dementia their mind isn't working correctly, they have no control over that.

Or as Pam had mentioned above, maybe she has a funny taste in her mouth. Also have her checked for acid reflex.

She may have a funny taste in her mouth and this could be as result of a stroke. See a Neurologist.