What could cause some alheimers patients more combative than others?

Perceptions: This depends on how the person is perceiving reality -- which depends on how severely ill s/he is, basic personality before alzheimer's, and existing other disorders too. People with dementia may misperceive the intentions of others because the the cortical loss impacts how they process incoming stimuli. If feeling threatened, etc, they can be more combative.

Many things: Time of day, level of fatigue, suspiiciousness, hunger, need to use the bathroom, inability to make oneself understood, frustration at any number of things...There are many reasons someone who cannot form new memories or use and understand language may become agitatd or aggressive.