From what I've read, they just seem to have grown apart, with each conducting their own affairs. I haven't sensed that it was a bitter thing to the extent were they grew to hate each other, just that they were happier living apart and occasionally staying together as a family. Seems a shame since by all reports, theirs was a love match and very passionate at the beginning (with Tsarevich Nicholas writing about how they were behaving very embarrassingly affectionate in front of him) and with Xenia hanging in to marry him despite her parents' reservations. I don't imagine MF was too thrilled, but since she'd already had one daughter divorce, and being a woman of the world, she probably wasn't shocked and just happy they didn't divorce themselves. There's a lot of information about their relationship (naturally!) in Xenia's biography by Coryne Hall which draws on various letters/diaries.
Subject: Grand Duchess Ksenia to Tsarevich Nicholas, 14 April 1894
Sandro and I are depressed that still nothing has been
decided about the wedding. You must understand, Nicky,
that it's a terrible bore and *quite impossible to
bear*! Please will you mention it to both Mama and
Papa when you return, as they will of course talk to
you about your wedding. You are still only *newly*
betrothed, but when *three months* have gone by you
will also begin to get tired, as I do, and to feel
that it simply *isn't possible to hold back any
longer*, especially when there isn't even any mention
of a wedding.
Subject: Grand Duchess Ksenia to Tsar Nicholas II, 27 May 1902
... As for Sandro [her husband], I have to admit that
it's very hard for me to be separated from him, I can
never get used to it.
Subject: Diary of Grand Duchess Ksenia, November 1911
[Sandro was often unfaithful. A nobleman named F. became infatuated with Ksenia, and they began an affair. At the same time, Sandro was having an affair with F.'s wife. These are entries from Ksenia's diaries in November of 1911. Ksenia is writing in Ai-Todor, Sandro's estate in the Crimea.]
13 November 1911
I feel depressed and I'm very annoyed. F. says that
it's time for him to go, he hates it here, that he's
sick of the whole set up and everything. I understand.
But we are also leaving at the end of the week, and
I'm trying to persuade him to leave with us. They have already begun to talk about us. He senses this, although I have said nothing to him. It's such a pity that he came this year, when everyone is staying here! The fault is ours, but on the other hand, why may we not have whom we choose?
29 November 1911
He went off to pack and at 11 o'clock left for good.
It's sad. but most of all I don't know when we'll see
each other again! We were together for over six
months!
30 November 1911
It's terrible how much I miss F.!