Another explanation is this one :
Around Jerusalem at the time, the method of burial was in the side of a hill or mountain. They usually did not bury the dead 6 foot under the dirt. These would be either naturally occurring caves or dug out especially for burials.
When the earthquake(s) happened--the main one with aftershocks, normally---this dislodged (literally) the sealed stones at the openings and the bodies inside. Gravity takes over and these bodies roll down the hill, into the streets of Jerusalem.
For those residents or anybody really, this caused the citizens to be very upset. But those bodies then did not get up and walk away. They remained dead.
Someone asked why you asked this of Jehovah's Witnesses? The answer is obvious. You wanted a correct answer based on research and knowledge. It is how we do things. Not by repeating what some clergyman has you recite.