Hi Simon,
I think all cases are different, however i have heard about this and seen it with friends parents - in one case an apparently amiable chap who pootled around, helped his wife as she was quite frail and was a loving husband for sixty years, suddenly started being violent towards her - and in another, a woman who had been very controlling as a mother suddenly became more loving, more accepting of his friends and totally different in personality. In her case i think it was the Victorian values and self control she had been raised with, and had maintained in her own relationship with her son were gone, and in the first case, frustration and fear - because it must be terrifying when your normal cognition is disconnected. Imagine if you opened your mouth to say something and the wrong words came out, if you couldnt process what others were saying to you, or you knew you were hungry or cold but couldnt help yourself or ask some-one else to help you resolve that.
My husband died of a stoke but it wasnt the first one and in between he was more prone to losing his temper, with himself as much as us, something i had never seen in him before. but it was frustration at the physical problems but also the fear caused by that loss of connection in the brain that created it.You can see your hand move but it isnt connected to your thoughts its as though its some-one elses hand - or you cant find your mouth with your spoon, you cant speak properly so people dont hear what you say, you cant remeber how something you used every day works.
I know stroke is not the same as dementia, but it is the same process i think of the normal neuro connections in the brain being damaged whether it is being done over time, or quite suddenly.