Hey Keith,
If all of the Change Owner activities came from the same source, you can pick one of the leads, and then go into its activity log and and the campaign which initiated it will be in a right hand column called campaign this. This will link to it and you can see why it was changed.
To remediate the Owner Change, you need a list of the previous owners. You'll need to create a smart campaign for each one with a smart list that says "Owner Was Change, Pevious Owner Was [Owner Name], Date of Activity between [timeframe]" You may also wish to designate New Owner, if these changes were all made to a single owner.
Once you have this, you'll want a Change Owner Flow step to the previous owner. This will need to be repeated for each owner from which leads were removed.
Hope this helps you out.