Usually from someone making a mistake during an appropriate merge and losing relationships or through someone making an incorrect merge.
The best way to track down what happened is through the Change Log. Start with the person that is the last one you still have. In the right hand column under Change Log, click on Show All. That should show you exactly what happened.
If you can't see what happened, post the person's ID so others can help you figure this out.
What to do about the marriage dates depends on how they got there. So many things could have happened, that is is probably best if you post the ID numbers of the affected individuals so others can give appropriate ideas of what to do to correct the problem.