those are actually the granite slabs and they aren't floating
the caskets beneath them are stressing the granite and you can see enough pressure in a few of the graves has caused the granite to lift some
the pressure comes from decomp gases and I imagine saturating the ground to that extent around the graves has allowed for some shifting and I think that level of water emtombment has probably flooded some of the older and/or cheaper constructed caskets
and the whole integrity of the grave and casket is compromised
from the looks of it, it appears this cemetery mandates the use of the expensive grave slab as I imagine this is in an area that could be expected to flood
but you can only do so much and people don't want to drive 50 miles to mamma's grave