Well, we're not seeing arrests of all the many developers and researchers who contributed to Tornado Cash simply in the interest of privacy as such (because that is not a crime), however, I presume that for this particular person there is some extra evidence establishing probable cause for the arrest.

For example, if it can be shown in court that he contributed to it with a knowing intent to facilitate privacy of illegal deals, that might be considered aiding and abetting "the concealment or disguise of the true nature, source, location, disposition, movement, rights with respect to, or ownership of, property, knowing that such property is derived from criminal activity", which is a crime under EU/Dutch AML law.