Some dogs run off with the leash, if the person isn't gripping tight, or doesn't have grip strength. Especially when the dog is young/not properly trained.
Also, I can imagine, after a long marriage, and when symptoms aren't constantly manifesting, it's probably hard to remember in every single moment that your spouse is no longer the person they were for so long. There's an ingrained assumption that operates when something happens that takes your attention elsewhere.
I think some people expect us to be programmed like computers, but humans don't function that way.
JMO