Hey Mandy, like Josh, I haven't seen this before. If someone (or something) is suspending the leads, it won't necessarily add a marketing suspended cause.
One way that might be helpful to find where this is being written, is either in the activity log of someone who is Marketing Suspended within the last 90 days (data value change details are removed after 90 days), or by finding where the Marketing Suspended field is used within the Admin > Field Management section.