I guess they thought -- 'hey, noone cares about this 1850 person here, let's just get rid of him and resell this plot!'
Actuaqlly it happens a lot more than you think. It just happened to be a 'blaque' plot. People think once they're buried, they'll be there forever and ever. Not so. Actually once a few generations have passed and no one even knows who you are -- they get rid of your plot. Kinda a taboo topic in the cemetary world no one likes to talk about. Notice how few '1700s' tombstones you find nowadays. When no one's looking, they get rid of them. They got sloppy and got caught. For pet cemetaries, i think they get rid of the bones after 40 years. Prolly just throw it in the trash.
Wanna be buried forever? Make your tombstone in antarctica or middle of africa.
r