My dad is in the late/end stage of dementia. Why does he “lock up”?

Certain types of dementia is associated with rigid muscles and 'stiff man syndrome' which was previously thought to ONLY be associated with Parkinson's Disease.

"We associate shaking and muscle stiffness with Parkinson's disease, but people with Alzheimer's can also have muscular problems. The muscles can become rigid, making it difficult to move, and there can also be a noticeable tremor."

Here's a link to an article discussing the matter in greater detail:

https://www.liftedcare.com/dealing-with-body-stiffness-and-aches-with-dementia/

The end of life journey differs greatly between people. Here is a link to a lot of useful information on that very subject:

https://www.alzheimers.org.uk/get-support/help-dementia-care/recognising-when-someone-reaching-end-their-life

Do you have hospice services on board for your dad now? You may want to ask his PCP for a referral and call around to a few hospice companies in your area to come by and evaluate him now. I've had very good experiences with hospice for both of my parents; they were able to keep my parents comfortable (but not over-medicated) at the end of their lives, and out of pain completely, easing their transitions greatly. They also provide hospital beds, supplies, medicines, CNAs to bathe your dad, and chaplain services to you all at no charge (Medicare pays 100% of their fees).

Wishing you all the best of luck with a difficult situation.

My Husband would have episodes where he would "freeze or space out" and one day the Hospice Nurse was visiting when it happened and she called it a Myoclonic seizure. She said it was nothing to worry about. But when I looked it up the description was muscle twitching. His was more like just freezing up. then I thought it might be that the muscles just lock rather than twitch.
If your dad is not on Hospice I would strongly suggest it. You will get a lot of help, support and advice as well as equipment, supplies and medications delivered to you.
Hospice will help you and your dad through the "final times"
((hugs))

While Alzheimer's and other dementias are often thought of as just memory problems what they really are is a group of brain diseases that eventually effect every system in the body, that's why they are fatal. Freezing us well known in Parkinson's and Parkinsonism, no doubt you are seeing something along those lines.
You may find it helpful to use Teepa Snow's hand under hand technique when assisting him, this can allow you to offer assistance without him hurting you because he's squeezing too hard

https://www.youtube.com/watch?v=iKT9YIVPREE