Typically it is because the teeth grew into the mouth in groups and have lived in the same environment for the same length of time. And now they have all gotten their cavities together too.

Children's second baby molars grow in and move forward to touch the first molars between ages 2 and 3. The toothbrush can no longer clean between these teeth, bacteria harbor in the tight space, and the decay process begins. It typically takes 12 to 24 months for this process to penetrate the enamel covering and begin to spread as a cavity inside the tooth. As you might expect, the cavities are happening to all eight molars at once.

Prevention: If we can disrupt, disorganize and flush the bacteria out of the space between the teeth, they never have time to colonize adequately to do damage. The most efficient way to do that is to floss daily. Halting the process once it has begun is harder than prevention, but it is still possible. First and most important is to start flossing. This will prevent any new cavities from starting. If the surface of the tooth is smooth where the cavity has begun, the flossing will clean properly and will probably stop progress. Additionally, using a fluoride toothpaste daily may remineralize the area.