It may be one (or more) of many reasons and it's hard to diagnose what forces are working on it. It might be something new or a culmination of little forces over a long period of time.
In any event, it sounds like a nice old Kay that may not be worth trying a do-it-yourself approach... too many unknowns
If it were my bass, I'd probably take the strings off, drop the soundpost and get it to someone who has a lot of experience with the old big basses.
Check for a loose bass bar - it's not so uncommon on those old Kay basses. Top really caves in when the bar lets go!
Take the top off to fix it. . .
These basses have a following so (in the abstract) it might be worth fixing. It is a serious undertaking so Mike is right, take it to someone who knows what they are doing. A very knowledgeable person is James Condino (Kaybassrepair.com.). That might be a long drive for you. If you were in the Boston area (still a long drive), Volker Nahrmann would be a good person to see.
I'd go on line before getting out the pallet knife and vinegar.
You should also expect a bunch of other things to be wrong.
I have checked the bass bar and it looks tight. The sound post is sitting on circular wooden pads and looks to be in the proper place. I have had the neck off this bass to re-glue. I am in North central BC Canada so shipping is not an option. I have repaired several violins but never had to work on a laminate top. Any advice would be appreciated. I am having a hard time finding information on this type of problem so any leads would help. I would like to thank you and all the other members for being so helpful.