Families and friends are often shocked when long marriages — including long, bad marriages — fall apart. You endured each other for 30, even 50, years, so why give up now?
I'd say it's for all the reasons that younger people divorce. The partners bore each other, harbor grudges, seek a different kind of life or fall in love with someone else. Being surprised at gray divorce strikes me as a form of ageism — as if one or the other of a couple (perhaps both) are somehow too old for disappointment, folly or hope.