As far as "why they're investigating" now (since they terminated the deal anyway)... this could end up bad for Adidas. People are suggesting morality clauses and such but those things usually address behavior outside of the relationship between the individual and the company (like the individual posting dumb <anti-something/some group> shit on social media.) This on the other hand is internal. Adidas is a German company and all western countries have laws that against this kind of behavior in the workplace. One common theme in these laws is that employers are required investigate all allegations of this type of behavior when they are made aware of it. If Adidas was found guilty of sweeping this under the rug heavy fines could be levied as well as civil suits filed by individuals affected.

So Adidas is in damage control mode now, hoping they can identify who the individuals are and pay them off before some sort of class action is launched. Maybe some middle managers get thrown under the bus (bad press now to avoid worse press later). Then they'll hope their "internal probe" satisfies regulators or reduces the fines they could get.