Why did my cat's top fangs fall out?

My 2 yr old cat's top two fangs have fallen out and I have no idea why. I brush her teeth at least three times a week. I took her to the vet yesterday who examined her and she is in perfect heatlh, immaculate teeth with no sign of infection, irritation or gum disease. he even pressed down into the gums and there was no pain, no blood or discharge. Basically the vet doesn't know why they could've fallen out. Has this happened to anyone else? Any veterinarians out there who might know what caused this? Is it somehow related to genetics? Could she possibly still have had her baby teeth and new ones will grow back? Maybe chewed on something really hard? the teeth aren't even chipped they just fell out and I'm a little worried that later in life her other teeth could fall out.