Well I would say that would not be all that uncommon for people with a lot of money, as much as they apparently have, to keep such a large amount of cash at home.
A 'sane' reason to keep that much money at home would be for bitcoin trading purposes, as it would probably be a bit of a hassle to have to go to the bank every time burt wanted to buy or sell large amounts of bitcoin, plus the fact that making a lot of cash deposits and withdrawals out of a personal account (he wouldn't have a business account because he was only doing it as a 'hobby') is a good way to get your bank account closed.
I am not sure what exactly Burt's wife's company does, not what the client does. Both the businesses names were redacted (and the locations

) in a letter that she wrote and published in a different section on her site. I would say there is a good chance that it would be difficult/impossible to determine exactly what they do.
I understand not using banks. I grew up next to the wholesale fresh-cut flower market that was open 4am-7am, for the delivery at 8am-9am, which was also before the bank hours. I think I mostly understand people dealing "cash only" and having suitcases full of them. But never at home! This is dangerous, you paint a target sign for yourself and the family. And I'm not taking about "jackbooted government flunkeys", just a regular old-fashioned violent crime. Neither BurtW nor his wife seem like random naives or Bitcoin cultists/evangelists.
I don't think there ever will be enough information available to determine the sanity of their actions. The best we can hope for is to determine legality.
Well, legality is rather boring (to me) subject. The legal details could change in an instant. I'm interested in "what they were thinking", what kind of operation they were running.