Even though dementia progresses differently for everyone, there are certain little quirks that a lot of my residents share. I don't know why, and can't pretend to. I'll leave that to the more educated people. I bet you can find them by googling.
But here's my incomplete list of Weird Stuff that you might notice your Wife/Mom/Grandpa doing that never used to happen:
1. Dressing in lots and lots of layers.
I have no idea why people do this, but a lot of them do. I can name at least 6 of my residents that I MUST get up and dressed by 6:45 am because if I'm too slow in arriving to cue them, they'll come out wearing several pairs of pants (some backwards, some zipped, some not), a robe with a t-shirt on top of it, no underwear on but some in their pockets, and one shoe. This is so common that at my facility, all the people's closet doors are lockable in order to prevent this, peeing into closets (men do that one) or rummaging through their neighbor's things.
2. Eating really, really slowly and maybe falling asleep during meals.
The sleep thing is probably because days and nights don't necessarily mean a lot to these folks, so they get tired whenever they're tired. Sometimes it's from meds, but not always. And the eating slowly is just generally because they don't seem to process hunger the same way and get distracted from tasks (like eating) easily. In more advanced stages, they're likely to have trouble chewing and swallowing, but softened or pureed foods can fix that.
3. Having trouble with physical sensations, especially with verbalizing them.
Someone with dementia might feel cold all the time, even when sweaty and piled with blankets. Or insist they're too hot even though they've got goosebumps. Or holler at me that the shower water is burning even when it's lukewarm. And then when I have them test it again a second later without me touching the temperature dial, it's fine or chilly. Or be hungry one second and then not a minute later. Saying they don't need to go to the bathroom but then peeing for ages once you finally get them to sit on the toilet. This is why caregivers usually go by physical cues for assessing pain and temperatures, and by the clock for eating/toileting.
4. Spontaneous emotional outbursts.
I think this one has to do with the part of the brain that used to inhibit emotional displays no longer acting as gatekeeper. That's why someone might burst into tears or laughter for no apparent reason. And might not know they're doing it. Usually pretty short in duration.
5. Difficulty with noticing surroundings.
This could be because their field of vision has narrowed, or because colors look the same to them now, or for some other biological reason I have no clue about. All I know is, if I want someone to sit down, and they don't seem aware that there really is a chair behind them, they do better if I take their hand and put it behind them onto the armrest, pat the seat loudly, put gentle pressure on their shoulders, and say "Bend your knees now. Bend your knees." Or sometimes even if the place I'm taking them is a few more steps away, I should still hold both their hands, face them, and back up to it that way so they don't change directions suddenly. I don't really understand this one either, but I've definitely noticed that pointing and saying "right over there" isn't good enough for most people to get them there. I need to take them all the way over there.