Someone must be using it inside a workflow. Lead Statuses are entirely up to you. Click on campaign inspector and look for lead status.
You can always run a filter on Lead Status=Active and use Change Data Value on it.
I would pull a smartlist of records with the Lead Status = Active. This will give you an idea of how large this data issue is in your instance. Once you've located those records, go into the lead activity log to determine what made that change. If it is a MKTO smart campaign it should show the name of the campaign populating the bad value so you can edit that campaign to stop this from occurring going forward. If you cannot see a campaign displayed on the right of the Data Value Change, it was likely changed by someone in the CRM side. Also, I would check with whoever manages your CRM system because Active could be a hidden status in SFDC but visible in MKTO.