Cairo -
I've been living with cats for OVER 25 years -- the only times I've ever had any of my cats exhibit this type of behavior was when there was an ILLNESS (kidney or bladder infection) or possibly stomach upset from change in diet, and on one occasion a bacterial infection of the bowels. OR when there was some sort of confrontational stress going on in the house. :unsure:
If your "home enviornment" hasn't changed in some way, and you haven't changed the brand of litter you use... you should get the cats checked at the vet. It sounds to me like they are TRYING to let you know that SOMETHING is WRONG ! ! !
Take a look around -- if you haven't changed something pretty drastically in their environment -- they MAY be ILL !!!
Hoping you find out soon!!!
"fancidots"