It's because her house was her domain and now your DDad is always home getting in the way and not knowing how to do things her way
It's normal and frustrating for both of them to acclimatise to this new setting
My DM had to sort of train my dad to think to do basic tasks
When my DDad retired from his city job where he commuted so was away most of the day she said he was completely doing her head in constantly being around
My DDad had a very well paid senior role and had little common sense in basic areas as my DM is very controlling so likes doing things her way around the house so she's had to learn to accommodate him and teach him 
They have lots of holidays abroad and UK city breaks each year so I guess that's fun for them to plan and do
She gave him tasks like chopping up veg, collecting bread each morning and the papers. He has his tasks like hoovering, painting etc
They go into London a lot on day trips and also see friends separately, DM is more sociable than DDad but he has connected with friends of his he didn't have time for.
Family visit them from all over and they're great hosts and have the space.
I'd say it took my DM at least a couple of years to get into the swing of having him around