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.
Another person has altered information in my family and totally altered the timeline with people that have nothing to do with my family . How do i stop this ? thankyou . Alan
@AlanFoster2 Here are a few articles from the Help Center that might be helpful:
How do I see what changes were made about a person in Family Tree?
How do I undo an incorrect change in Family Tree?
How do I undo a merge in Family Tree?
How do I correct parent-child relationships in Family Tree?