These kinds of cats are quite sensitive to sudden pH change, as informed by a well known whip tail breeder up here in Auckland....can be sudden or over 2-3days apparently. Sorry about your loss.
As for the Kulli killing your cats, that's total c**p, they are placid as and have over 8 of them and have never hurt anyone, I rarely see them. Unless, they were swimming about and stressed the cats too much? Bit far fetched maybe? My 5c thanks.