There's always a reason.
I am the "dump-er" in such a scenario. My supposed best friend since we were in our early 20s (went through our parents dying, other upheavels) had a horrid marriage starting at about age 40. Not physically abusive but he was always complaining, suspicious, etc. and therefore all of our outings / plans had to be coordinated around his needs, she would cut short an outing of, say, antique shopping, to get back to make his dinner, she complained endlessly about him.
He got a rare form of early dementia at age 60 and she (in her early 50s at the time) had a hellish year and a half till he died. During that time period I used a LOT of my annual leave to help her, sitting with him while she went to opticians, that sort of thing, going with her to hospital when he was locked briefly in a psychiatric ward, you name it.
So he dies and after the funeral, etc., I of course offered several times to come over and help her sort his things or just keep her company. I guess deep down I figured our friendship would go back to what it was the first two decades, doing fun things together, etc. but she kept being unavailable. She has good vision for projects and I'd asked if sometime, when convenient, she could come to mine and give me some advice on re-arranging the kitchen. Never happened.
Turns out that a month or so after the death, she hired a man to refurbish the bathroom and started a relationship with him.
She had no time for old friends but plenty of time to integrate herself into his family, help plan his daughter's wedding, make play equipment for his grandchildren, help him in his business (he's a self-employed builder), etc. etc. I felt phased out once AGAIN for a man and just stopped e-mailing, etc. She reached out a few times asking what was wrong and I just said "nothing, just so busy with work these days." There seemed to be no point to get into details.
That was pre-Covid. It stung at first and I miss being able to reminisce about certain things (she's one of the few people left who knew my parents, for example) but c'est la vie. Plenty of fish in the sea.